Development of the Dermal Bones in the Skull
of Lepidosteus osseus.
By
F. J. Aumonier, M.Sc,
University College, London.
With Hates 1-8 and 2 Text-figures.
INTRODUCTION.
THE primitive features of the ' Ganoid' fishes have given this
group a position of considerable importance. Therefore it
was decided that the work described below would be amply
justified for the sake of the observations alone, quite apart
from the deductions which have been drawn from them.
However, the main object of this work was an attempt to
find out whether the complex names of bones used by Stensio's school (post-rostro-nasal, inter-temporo-supra-temporal,
&c.) were justified. That is to say whether some of the
fusions which Stensio claims to have occurred in phylogeny
can be found in ontogeny. Owing to the close relationship
existing between certain of the dermal bones and the canals
of the latero-sensory system it seemed necessary to devote
particular attention to the study of this relationship during
development. The nature of the association between bone
and canal is unknown. It may well be that the presence of the
sensory organs in the skin affects the underlying mesenchyme
directly, and induces it to lay down bone. Alternatively, the
same factor which determines the position of the sense organs
might simultaneously initiate a longer process which leads to
the formation of bone.
Pehrson (1921) published an account of the development of
the skull of A mi a; this rendered it very desirable to discover
how the development of L e p i d o s t e u s compared with its
less specialized relative. However, it is obvious that the great
development of the rostrum in L e p i d o s t e u s must bring
NO. 329
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2
F. J. AUMONIEE
important functional demands upon the structure of the skull.
Therefore allowance must be made for modifications both in
structure and development, which have arisen chiefly to satisfy
mechanical necessities.
Agassiz (1833-43) gave the first detailed description of the
adult skull of L e p i d o s t e u s . The accuracy of the figures and
text have not been equalled. Indeed it would seem as if later
workers have concentrated on specialized aspects of morphology, and thereby failed to appreciate the situation as a whole.
Parker (1882) described the development of the skull, but
as at that time the importance of the lateral line canal was not
understood, the value of his work is reduced. Moreover, the
dermal bones are not so fully dealt with as are the cartilages and
cartilage bones.
Allis (1905) in a general study of the latero-sensory canals of
fishes, dealt with L e p i d o s t e u s ; but, though he gave a
thorough account of the canals and pores, yet the dermal bones
received scant attention.
Veit (1907-11) described the chondrocranium in detail and
extended his work with a thorough study of nerves and bloodvessels, but again there was no reference to dermal bones.
Eegan (1923) gave a short paper on the skull of L e p i d o s t e u s . In this case the adult skull alone was described,
though not in such intimate detail as had been given by
Agassiz.
Mayhew (1924) published a somewhat longer paper on the
adult skull of L e p i d o s t e u s p l a t o s t o m u s . His work is
of a more general nature than that of any preceding author
since the time of Agassiz, but he does not deal with development.
Goodrich (1930) gives several figures of skulls of L e p i d o s t e u s , but he was more concerned with special problems
of morphology than with the animal in particular.
Gregory (1933) deals very briefly with L e p i d o s t e u s and
the same remarks apply to his text and figures as to those of
Goodrich.
Hammarberg (1937) published a very extensive paper on the
skull of L e p i d o s t e u s p l a t o s t o m u s . This is by far the
best work since that of Agassiz, and is well illustrated. There
SKULL OF LEPIDOSTEUS
3
are, however, certain differences between the observations of
Hammarberg and those of the present author.
MATERIAL AND METHODS.
All except one of the specimens o f L e p i d o s t e u s o s s e u s L .
referred to in this paper were purchased from Turtox Ltd., of
America, and had been fixed in formalin. The preservation was
tolerable, but a certain amount of shrinkage had taken place.
Three specimens were examined and reconstructed: 19 mm.,
25 mm., 38 mm.
The following stages were examined but not reconstructed:
6 mm., 12 mm., 13 mm., 14 mm., 15 mm., 16 mm., 17 mm.,
22 mm., 11 cm.
The specimens were decalcified in a solution of 4 per cent,
nitric acid in 90 per cent, alcohol. After decalcification three
photographs were taken of each specimen (one dorsal, one
ventral, and one lateral view). The overall length and the distance from tip of snout to centre of the lens of the eye were
measured and recorded.
The fish were then cut into serial transverse sections, 10/x
thick for those of less than 25 mm. overall length and 20/i thick
for larger individuals. Eeconstructions were made at a magnification of 100 diameters for the smaller, or 50 diameters for
the larger, fish. Two views of each fish were reconstructed, one
a plan, the other an elevation. In every case the plans represent
the fish viewed from its dorsal surface, while the elevations
show the left side of the animal. The reconstructions are all of
the type of architectural drawings; no attempt is made to
obtain perspective. This leads to a somewhat artificial appearance, but it enables accurate measurements to be taken with
ease.
The method employed was as follows: a microprojector was
set up so as to give the required magnification (either 50 or
100 diameters). To obtain a 'plan' a sheet of 1 mm. squared
graph paper was marked with a base line, every millimetre
division of which represented the thickness of one section. The
outline of the animal and the position of the inner and outer
edges of all bones are plotted with the aid of proportional
4
F. J. AUMONIBE
compasses set to divide by two. The distance between the two
sides of the fish on a given section being set upon the outer
ends of the compasses, they are then transferred to the graph
sheet and the distance between their inner ends (i :>. half that
of the outer) is then laid off on each side of the ' base line' along
the line of the appropriate section.
This has the disadvantage of masking any slight asymmetry
if it be present, but as such asymmetry would be devoid of
morphological significance it was decided to accept this limitation for the sake of speed and ease of working. If the microtome
cannot be relied upon to cut sections with reasonable accuracy
there will be a longitudinal distortion; as, however, all structures will be distorted equally, this also is of small account.
It can most readily be detected by the shape of the orbit; if
the latter appears on the graph as a circle, then for practical
purposes there is no distortion.
The elevation is more difficult to obtain, but it is, if anything,
more accurate than the plan. The photograph of the lateral
view of the fish is projected on to a sheet of graph paper and
the size of the image is adjusted till the projected photograph
has the same magnification as was used in plotting the sections.
Taking a simple case, suppose the lens on the left side begins on
section 240 and ends on section 260, then its centre can be
assumed to be on section 250; then, if the sections are 20^.
thick, we use a magnification of 50 diameters for plotting, and
so the photographic image is adjusted till the distance between
the tip of the snout and the centre of the lens is 250 mm. In
order that the vertical lines of the graph shall correspond to the
plane of section in a dorso-ventral direction a third point of
reference is required. Suppose the lower jaw first appears on
section 83. Then our graph sheet is moved about till the tip of
the nose lies on division 1, the centre of the lens on division 250,
and the tip of the lower jaw on division 83. The orientation is
now as exact as may be, and the outline of the fish is then traced
in pencil on the graph paper. Other easily identifiable structures
such as the lens, operculum, nostrils, gape, &c, are also traced.
Owing to the flexibility of the floor of the mouth there is bound
to be a considerable discrepancy between the depth of the fish
SKULL OF LBPIDOSTBUS
5
before embedding and after cutting. For this reason the outline
of the dorsal surface is used as a datum line and is at once
inked in. The image of the sections is projected in the same
manner as in making the plan; but, instead of using proportional compasses, the distance between the dorsal surface and
the upper and lower limits of all bones is transferred from the
image of the section to the graph by means of dividers. If
the ventral surface be plotted thus, it will be noticed that the
plotted outline is always more dorsal than the outline obtained
from the photograph; and this difference increases towards the
posterior end.
The important stage of 25 mm. was the only exception to the
above procedure. This was a series of 10/z. sections very kindly
lent by Dr. G. E. de Beer. As there was no photograph of this
specimen, the elevation was obtained by assuming that the
parasphenoid was a straight line. After a preliminary outline
had been obtained, the outline of the dorsal surface was faired
by eye till it assumed a shape roughly similar to that of one of
the adult skulls which was available. The parasphenoid was
then replotted, using the revised dorsal surface as a datum line.
The outline of the head anterior to the parasphenoid was reconstructed by Pusey's method.
I shall take this opportunity for thanking Professor D. M. S.
Watson and Dr. E. A. Fraser, under whose supervision the work
was done (at University College, London), for much helpful
criticism, and Dr. G. R. de Beer, for the loan of the 25-min. stage.
Dr. T. S. Westoll has also given me good advice on many points.
Mr. H. K. Pusey very kindly showed me how to use his system
of reconstruction, which was essential for the elevation of the
anterior end of the specimen lent by Dr. de Beer. I am indebted
to Mr. J. R. Thomas for the photomicrographs and for the
photographs of the specimens; these photographs were an
essential part of the method of reconstruction. Mr. S. R. Scarfe
has advised me in the preparation of sections, and Miss Joyce
Townend in the preparation of the figures.
The question of bone nomenclature must next be considered.
Owing to the rapid advances now being made in the study of
morphology, it is desirable to present observations in such a
6
P. J. AUMONIBR
manner that the reader can readily separate the facts from the
theories. Therefore the following method will be adopted. The
development of the skull will be described bone by bone, each
element having a separate paragraph. The paragrapl will be
headed with the original French terms used by Agassiz, and the
name which the present author believes to be correct will follow
in brackets. These latter names will be used in the text and on
the figures. In the discussion the bones will be dealt with in
groups and the author will attempt to justify his choice of
terms.
In order to assist any one who may wish to follow more
modern authors, the following table gives the French terms of
Agassiz and their modern equivalents. In most cases the names
of some of the modern authors are given in brackets. This list
does not pretend to be complete, but it may be a useful guide.
Terms used by Agassiz.
Frontaux principaux.
Frontaux anterieurs.
Modern Terms.
Frontal.
Prefrontal.
(Premaxilla (Gregory).
( Ethmonasal (Parker).
Nasaux.
VNaso-premaxillary (Mayhew).
Frontaux posterieurs.
Sphenotic (Regan, Mayhew).
Basilaire.
Basioocipital.
Parietaux.
Parietal.
Sphenoide principal.
Basisphenoid.
I Dermo-supra-occipital (Gregory).
Oooipitaux superieurs.
( Dermo-ocoipital (Mayhew, Regan).
(.Post-parietal (Goodrich).
Oocipitaux externes.
Tabular (Mayhew, Gregory).
Oocipitaux lateraux.
Exoooipital (Mayhew, Regan).
Grandes ailes du Sphenoide. Pro-otic (Regan, Mayhew).
/ Supra-temporal (Mayhew).
Mastoidiens.
\ Pterotic (Goodrich, Regan, Gregory).
Rochers.
Epiotic (Mayhew, Parker, Veit).
Ailes orbitaires.
Alisphenoid (Mayhew).
Ethmoiide cranien.
Orbitosphenoid (Mayhew).
Vorncr.
Vomer (Regan, Mayhew).
I Premaxillary (Gregory, Regan, Goodrich).
Intermaxillaires.
1 Nasopremaxillary (Mayhew).
Maxillaires superieurs.
Maxillary chain.
Jiigaux.
Preorbitals (Mayhew).
SKULL OF LEPIDOSTEUS
Os mobiles du nez.
Palatins
Transverses.
Pterygoides.
Os cane.
Caisse.
Operculaires.
„ ,
,
Preopercules.
r
Tympano-malleaux.
' a, b, c '
'p, p.'
Sous-orbitaires ( f a n )
7
/These include Mayhew's dermethmoid,
I median prenasal, and lateral prenasal.
{ Goodrich's ethmoid, nasal, and adnasal.
I Gregory's nasal, adnasal (or antorbital) and
V dermethmoid (or rostral).
f Ectopterygoid (Regan).
\ Palatopterygoid (Mayhew).
Pterygoid (Ectopterygoid) (Gregory).
Metapterygoid.
Quadrate.
Sympleetic (Mayhew, Regan).
Opereular (Mayhew, Regan).
(Preoperoular (Mavhew, Goodrich).
\T ,
,
.-1 .
I Interoperculum (Regan).
(Interoperculum (Gregory).
| Interoperculum (Goodrich).
{ Praeoperculum (Regan).
Mentioned by Mayhew.
Not mentioned by any other author.
Circumorbitals.
Before dealing with the dermal bones of the skull, it will be
necessary briefly to describe the lateral-line sense organs, and
also to give some account of the chondrocranium. The individual
sense organs of the latero-sensory system of L e p i d o s t e u s
are developed early. They were found in the 12-mm. stage, and
the only difference between the organs found in thisfishand
those of the 19-mm. stage was that of size. The appearance of
the organs in a transverse section of the fish's head is very
similar to that of a sagittal section of an onion. Each organ
consists of a bundle of thin columnar cells, which extend from
the body surface to the basement membrane. The bundles are
almost spherical, but in the centre of each a narrow roundbottomed depression of the body surface extends downwards
towards the centre of the organ. This depression varies in
depth from one organ to another, but is generally from one-half
to one-third of the total depth of the organ. When the canals
are formed the organs become much larger and flatter, while
the central depression vanishes.
The lateral-line organs can be distinguished from the other
cutaneous sense organs by the central depression and by their
larger size; also by the fact that when the reconstruction is
8
F. 3. AUMONIER
made they form smooth continuous lines. Their number is
subject to some individual variation, as the following table will
show:
Number of organs in supra-orbital line
.
Number of organs in infra-orbital line
Number of organs in operculo-mandibular line
19 mm. 25 mm. 38 mm.
28
21
25
32
32
25
25
24
21
There is no indication of any increase in number after the
19-mm. stage.
Text-fig. 1 is a photomicrograph of an organ from the supraorbital line of the 19-mm. fish; Text-fig. 2 is of an organ in the
canalized portion of the infra-orbital line of the 38-mm. fish;
both taken with Zeiss 3-mm. apo. objective N.A. 1-4 and dry
condenser (below).
TEXT-FIGS.
1-2.
Photomicrographs. Both are taken with Zeiss apochromatic 3 mm.
N.A. 1-4 and dry condenser.
Fig. 1.—Organ from supra-orbital line of 19-mm. stage.
Fig. 2.—Organ from canalized region of infra-orbital line of
38-mm. stage.
The chondrocranium has been so well described by Veit
(1907 and 1911, and Hammarberg, 1937) that only the briefest
account of one stage (38 mm.) will be given here. Fig. 22,
PI. 7 is a dorsal, fig. 23, PL 7 a lateral, and fig. 24, PI. 8 a
SKULL OFLEPIBOSTBUS
9
ventral view of the chondrocranium of the 38-mm. fish. Fig.
24, PL 8 is a true ventral view and not a dorsal view of
elements on the ventral surface as is the case in the palatal
reconstructions of the dermal bones. Infig.22, PL 7 a small
section of the supra-orbital bar on the right hand side has been
removed to show the articulation of the palatopterygoquadrate
cartilage with the basitrabecular process. The trabecular horn
on the same side has also been removed to show the palatal
process of the palatopterygoquadrate cartilage.
Infig.23, PL 7 the trabecular horn is only partly visible in
two places owing to the presence of the palatal process of the
palatopterygoquadrate. On the dorsal surface the left-hand
side of the posterior fontanelle and of the taenia tecti media
posterior are visible. The anterior fontanelle is hidden. In
fig. 24, PL 8 the right-hand side has had the palatopterygoquadrate and hyomandibula removed, in order to show the
cornu trabeculae and the basitrabecular process. The further
details of the arrangements of the cartilages can be seen from
a study of the three figures showing reconstruction of cartilage
and bone (figs. 25, 26, and 27, PL 8), and from the numerous
transverse sections of the different stages.
The following is a description of the bones.
Os Mobiles du Nez. (Eostrals, nasals, and antorbitals.)
There are three pairs of these, all of which are superficial.
The most anterior pair lies on the ventral surface anterior to
the olfactory organ. It was only seen in the 38-mm.fishand
was very small; owing to the oblique angle at which the skin of
the snout was cut, it was impossible to identify the ethmoid
commissure, but a few lateral-line organs (not shown in the
figures) seem to be associated with this anterior pair of bones.
The bones are shown in plan infig.14, PL 6, in elevation in
fig. 13, PL 6, and in transverse section infig.12 a, PL 5. Their
position relative to the rostral cartilage is shown in figs. 26
and 27, PL 8.
The next pair lies on the dorsal surface. In the 38-mm. stage
the bones cover the top of the olfactory organs, extending from
a point vertically above the anterior end of the rostral cartilage
10
F. J. AUMONIBE
(septum rostri of Veit) to a position abreast of but mesial to the
posterior nostrils. The right-hand bone has three, the left five,
organs of the supra-orbital line associated with it. In the 38-mm.
stage they are illustrated in fig. 13 (elevation), fig, 14, PI. 6
(plan), in transverse section in fig. 126, PL 5, and in relation to
cartilage infig.25, PI. 8 (plan) andfig.26, PI. 8 (elevation). These
bones were first seen in the 25-mm. specimen. They were very
small and lay near the middle of the inner edge of the olfactory
organs, in intimate association with the most mesial pair of organs
of the supra-orbital lines,fig.6, PL 2 and fig. 7, PL 3 (elevation),
fig. 8, PL 3 (plan), and fig. 6a, PL 2 (transverse section).
The third pair has only been seen in the 38-mm. stage. The
bones lie laterally to the posterior end of the olfactory organ ;
each bone carries two organs of the infra-orbital line. Fig. 13,
PL 6 shows the bones in elevation, fig. 14, PL 6 is the plan;
the transverse section is seen in fig. 12 c, PL 5, and the relations
to the septum rostri in figs. 26 and 27, PL 8.
It should be noted that no signs of cartilaginous olfactory
capsules could be found at any stage.
Intermaxillaires.
It was found that these were really the anterior ends of the
bones called 'nasaux', and even in the 17-mm. stage there was
no sign of a separate origin.
N a s a u x . (Premaxillaries.)
These bones were first seen at the 14-mm. stage. In the
19-mm. fish they are of large size. They lie one on each side of
the septum rostri, and are thin sheets of bone which are flat
and lie vertically at their anterior ends; but, immediately
posterior to the anterior nostrils, they become strongly curved
in transverse section (with the concavity facing laterally). At
about the middle of their length a lateral shelf is formed and
the olfactory nerve runs ventrally to it. In later stages the outer
edge of the shelf extends ventrally and fuses with the main
bone ventral to the olfactory nerve, which is then enclosed in
a bony tunnel. The posterior extent of the bones varies with the
size of the fish, but though in the 19-mm. stage there is a gap
SKULL OF L E P I D O S T E U S
11
between the posterior ends of the premaxillaries and the anterior
ends of the frontals, yet these two pairs of bones overlap in the
25 mm. and all subsequent stages. The premaxillaries always
lie very close to the septum rostri, but they do not come into
any kind of association with the organs of the supra-orbital line
until the horizontal shelf mentioned above has been established
for a long time. In the 19-mm. stage they can be seen in
figs. 1 and 2, PI. 1 (elevation and plan), figs. 1 a and b, PL 1
(transverse section). In the 25-mm.fish,fig.6, PL 2 and fig. 7,
PL 3 are elevations,fig.8, PL 3 is a plan, and figs. 6 a, b, c, and
d, PL 2, are transverse sections at different stations. Fig. 6 d,
PL 2 shows the overlap of the premaxillaries and frontals. The
illustrations of the 38-mm. specimen show the premaxillaries
in fig. 12, PL 4 (elevation), fig. 14, PI. 6 (plan), figs. 12 c, d,
e, f, and g, PL 5 (transverse sections). Their relations to the
cartilages are shown in the sections and in figs. 25 (plan) and
26, PL 8 (elevation).
F r o n t a u x P r i n c i p a u x . (Frontals.)
These were first observed in the 16-nnn. stage. They were
small, slightly concave plates lying below the organs of the
supra-orbital line for a distance of three consecutive organs just
above the orbit. The bone was slightly thicker in the region of
the organs than in between them. It was, however, continuous
throughout, and was thickest under the middle organ, thinner
anteriorly, much thinner posteriorly. In the 19-mm. fish the
frontals were quite large bones. Their relationships with the
premaxillaries have been described above. Posteriorly they
extend slowly and do not reach far beyond the orbit till the
38-mm. stage when they carry 12 organs. They lie very close
to the supra-orbital cartilage, slightly lateral to it. In the 38mm. stage their posterior ends are flared outwards so as to pass
outside the antero-lateral edges of the posterior fontanelle.
They lie very near to the cartilage at all stages. Fig. 1, PL 1
is a lateral and fig. 2, PL 1 a dorsal view of these bones in the
19-mm. specimen, the transverse section is seen in fig. 1 c, PL 1.
In the 25-mm. stage they are shown in elevation in fig. 6, PL 2
and fig. 7, PL 3, in plan in fig. 8, PL 3, and in transverse
12
F. J. AUMONIBR
section in figs. 6 d, e,f, and g, PI. 2. The figures for the 38-mm.
fish are 12, PI. 4; 14, PL 6; 12 g, h, i,j, k, PI. 5.
P a r i e t a u x . (Parietals.)
The posterior end of the skull develops somewhat later than
the anterior, and these bones were first seen in the 38-mm. stage.
They are simple sheets of bone lying near the mid-line on the
dorsal surface and extending from the posterior end of the orbit
to the middle of the auditory capsule. Their antero-lateral
edges just overlap the postero-mesial borders of the frontals.
Their mesial borders do not quite reach the lateral margins of
the taenia tecti media posterior, thus the posterior fontanelle
is still incompletely roofed at this stage. The posterior ends of
the parietals do not reach the posterior end of the auditory
capsule; the lateral edges do not descend very far down the
sides of the cranium. Fig. 12, PI. 4 gives the elevation and fig.
14, PI. 6 the plan of the parietals; the sectional view is fig.
12fc, PL 5, and the relations to the cartilage can be seen in
figs. 25 (plan) and 26, PL 8 (elevation).
The posterior row of roofing bones and the shoulder girdle
will be dealt with later.
M a x i l l a i r e s S u p e r i e u r s . (Infra-orbitals.)
The first indication of these was in the 25-mm. stage. In this
animal they were small gutters immediately underlying each
organ of the infra-orbital line. The series began with the first
organ posterior to the posterior nostril and extended for five
organs in succession. The mesenchyme condensations in which
they were found were in close contact with the epidermis in the
region of each organ. The bones appear to develop from before
backwards, the posterior elements being very small. A few
teeth had become attached to the larger bones. A great number
of teeth have no attachments of any sort till much later.
In the 38-mm. specimen, the bones had become more elaborate and had increased in size. The most anterior element of
each row had fused with the one behind it, while the gaps
between the succeeding bones were reduced to narrow slits.
The infra-orbital line ran through the first five elements as a
SKULL OF LEPIDQSTEUS
18
closed canal with pores at each suture. Many teeth were
attached to the bones (fig. 6, PL 2; figs. 7, 8, PL 3;fig.6 b,
PI. 2 ; figs. 13, 14, PL 6; figs. 12d, e, and/, PI. 5).
'a, b, c ' (Maxillaries.)
It has not been possible to establish the existence of three
bones in this region. Only one could be found, and this was
also the case in all four of the adult skulls studied. The single
bone is present in the 17-mm. and all subsequent stages: it lies
in a fold of skin between the upper margin of the gape and the
lower margin of the labial groove. Its anterior end is between
the fifth and sixth organs of the infra-orbital line (but far
ventral to them). It reaches its maximum relative size in the
25-mm. stage and is relatively smaller at 38 mm. In every case
it bears a few teeth; three or four small specks of enamel were
found on this bone in a large adult skull. These might be the
remains of teeth formed in early development which had
remained vestigial (figs. 1, 2, 2b, PL 1; fig. 6, PL 2; figs. 7, 8,
9, PL 3 ;figs.6b and c, PL 2; figs. 13, 14, PL 6;fig.12g, PL 5).
S o u s - o r b i t a i r e s . (Infra-orbitals.)
There is no sharp division between the ' sous-orbitaires' and
the 'maxillaires superieurs'. Indeed there may not be any of
them present in the 38-mm. fish. If they do exist, they are the
two most posterior elements of the chain labelled infra-orbitals
in fig. 13, PL 6 (those in which the infra-orbital line is a deep
groove but no longer a closed canal). They are superficial and
the infra-orbital hue passes over them. Lamellae of bone rise
up beside the groove.
It may be mentioned here that the cheek plates are absent
in the 38-mm. stage.
P r e o p e r c u l e . (Preopercular.)
This lies in the opercular flap and is quite superficial. It was
first seen in the 25-mm. fish. The operculo-mandibular line
runs along it, but only two organs come into intimate relation
with the bone. In the 38-mm. specimen the bones are much
14
F. J. AUMONIBR
larger, and each one carries six organs (fig. 6, PI. 2; figs. 7,
8, (> h and i, PI. 3; fig. 12, PI. 4; fig. 17, PI. 7; figs. 12 j and k,
PL 5).
It will be seen that the preopercular covers the ventra1 and
posterior regions of the hyomandibula (fig. 26, PI. 3).
O p e r c u l e s . (Opercular.)
These bones were first seen as small thin sheets in the 38-mni.
stage. They lay in the opercular flaps, slightly above and behind
the postero-dorsal corner of the 'preopercules', between the
latter bones and the supratemporals. They have no connexion
with any part of the latero-sensory system (fig. 12, PI. 4; fig.
17, PI. 7).
S o u s - o p e r c u 1 e s. (Subopercular.)
This again was first seen in the 38 mm. It lies ventrally to
the opercular, and the same remarks apply to it as to the
opercular (fig. 12, PI. 4; fig. 17, PL 7).
M a s t o I d i e n s. (Supra-temporals.)
In the 25-mm. stage these bones were quite small but they
had a recognizable association with the infra-orbital line (each
bone carried one organ). They are superficial, and are situated
dorso-laterally, directly above the preopercular. In the 38-mm.
fish they are larger and carry four organs of the infra-orbital
line. They are lateral to and coextensive with the parietals:
their antero-dorsal edges overlap the postero-ventral margins
of the frontals. The antero-ventral borders of the supratemporals do not quite reach the ventral margins of the supraorbital cartilages. At about the middle of their length the
ventral edges of the supra-temporals overlap the articulation
of the hyomandibula with the side of the auditory capsule.
Their elevation is shown in fig. 6, PI. 2, and fig. 7, PI. 3 for
the 25-min. and fig. 12, PI. 5 for the 38-mm. stage. In plan,
fig. H, PI. 8 gives the situation at 25 mm. and fig. 14, PI. (! at
38 mm. The transverse sections are shown in fig. (ii, PI. 3.
The relations to cartilage appear in ligs. 25 (plan) and 26,
PI. 8 (elevation).
SKULL OFLEPIDOSTBUS
15
Palatal Bones.
V o m e r s . (Prevomers.)
The prevomers appear at the 14-mm. stage, and in the
19-mm. specimen have attained a reasonable size. They are
immediately ventral to the septum rostri. Their anterior ends
are just posterior to the anterior nostrils, while posteriorly they
pass one on each side of the parasphenoid. In the later stages
they are much bigger: they grow posteriorly and laterally.
Their inner edges almost meet in the mid-line just ventral to
the parasphenoid. They bear many teeth. They are always
tightly applied to the ventral surface of the septum rostri.
In the 19-mm. specimen they are shown in figs. 2 (plan), 5
(elevation), and 1 a (transverse section), PL 1. For the 25-mm.
stage the figs, are 8 and 9 (plans), 10 (elevation), PI. 3, figs.
6 b and c, PI. 2 (transverse sections). The prevomers of the
38-mm. fish are shown infig.15, PI. 6 (elevation),fig.17, PI. 7
(plan), figs. 12 d, e, f, PI. 5 (transverse sections). The neighbouring cartilages are seen in figs. 26 (elevation) and 27,
PL 8 (ventral view). It must be mentioned that fig. 27, PL 8
is the only exception to the practice of showing the palatal
bones as viewed from the dorsal surface.
P a 1 a t i n s. (Palatines and ectopterygoids.)
Agassiz regarded the ectopterygoid as the palatine and
thought that the thin tooth-bearing bone which lies ventrally
to its anterior end ('superficial palatine' of Parker) was an
accessory tooth-plate, comparable to those in the oesophagus.
These superficial palatines were found in all stages from 14 mm.
onwards. In the 19-mm. specimen the posterior half of the
palatines lies just ventrally to the anterior ends of the ectopterygoids, but in later stages the ectopterygoids extend
anteriorly till only the extreme tips of the palatines extend
beyond them. The palatines are illustrated in figs. 2 (plan),
5 (elevation), 1 a and b, PL 1 (transverse sections), for the
19-mm. fish. Figs. 8 (plan), 10, PL 3 (elevation),fig.6 c, PL 2
(transverse section) show the 25-mm. specimen, whilefig.16, PL
16
F. J. AUMONIBR
6 (elevation), fig. 17, PL 7 (plan), and fig. 12/, PI. 5 (transverse
section) are of the 38-mm. stage. The cartilages and their
relations to the palatine are shown in figs. 26 (elevation) and
27, PL 8 (ventral view).
The ectopterygoids are also developed early, making their
appearance in the 14-mm. fish. Their relationship to the
cartilaginous skeleton is interesting. In the centre of the snout
lies the long slender cartilaginous septum rostri. In the 38-mm.
stage the cornu trabeculae have fused with the septum rostri
for the posterior third of its length, for the middle third of the
course of the septum rostri the cornua are free forwardly projecting rods. Immediately ventro-laterally to the cornua lie
the slender palatal processes of the palatopterygoquadrate
cartilages. Tightly pressed to the ventral surfaces of the palatal
processes are the ectopterygoids. In this stage they are thick
cancellated masses which must have an important role in
stiffening the snout. At the level of the anterior end of the
brain case the palatopterygoquadrates take on a different
form. They are here deep girders, the dorsal edges of which are
nearer the mid-line than the ventral. The antero-ventral cornua
of the palatopterygoquadrates project slightly forward and
form the articulation for the lower jaw (hence the lengthy name
of palatopterygoquadrate is justified). At the point where the
palatal process joins the main girder, the ectopterygoid becomes
a broad thin sheet and is tightly pressed to the median face of
the palatopterygoquadrate. The bones are shown in the 19-mm.
stage in figs. 2 (plan), 5 (elevation), and 1 b and c (transverse
sections), PL 1. The illustrations for the 25-mm. fish are fig.
6, PL 2 (elevation), fig. 8, PL 3 (plan), and figs. 6 c, d, e, / ,
and g, PL 2 (transverse sections). The 38-mm. specimen is
shown in fig. 16, PL 6 (elevation), fig. 17, PL 7 (plan), and
figs. 12/, g, li, i, j , and k, PL 5 (transverse sections). The
relations to the cartilages are best seen in the transverse
sections, but figs. 26 (elevation) and 27, PL 8 (ventral view)
may be useful.
P t e r y g o i d e s. (Endopterygoid.)
According to Agassiz the bone since called by Mayhew the
SKULL OF LBPIBOSTBUS
17
metapterygoid was the ' pterygoiide', while the mesopterygold
of Mayhew was a mere accessory plate. The cartilage bone does
not concern us, but the accessory plate exists as a thin splint
running along the dorso median edge of the pterygoid portion
of the palatopterygoquadrate for a short distance below the
posterior half of the orbit in the 38-mm. stage. Fig. 12, PI. 5
(elevation),fig.17, PI. 7 (plan),fig.12 j , PL 5 (transverse section), figs. 26 (elevation), and 27, PI. 8 (ventral view).
Sphenoi'de P r i n c i p a l . (Parasphenoid.)
This bone is first seen at the 14-mm. stage. In the three stages
which have been reconstructed, it is of great length. In the
19-mm. fish it already extends from the posterior third of the
prevomer to the extreme posterior end of the skull. In the later
stages it extends steadily forwards. Pigs. 2 (plan), 5 (elevation), and 1 a, b, and c, PI. 1 (transverse sections) show the
parasphenoid in the 19-mm. stage. It has not been shown in
full in the 25-mm. stage but figs. 8 and 9, PL 3 show the posterior and anterior ends respectively in plan, whilefig.6, PL 2
and fig. 7, PL 3 show the posterior end in elevation. In the
38-mm. specimen the parasphenoid is seen in elevation in fig.
15, PL 6, in plan infig.17, PL 7, and in transverse section in
figs. 12/ to k inclusive, PL 5. Its relations to the chondrocranium are seen in figs. 26 (elevation) and 27, PL 8 (ventral
view). The parasphenoid is always pressed closely to the
ventral surface of the chondrocranium, from the septum rostri
to the occipital region except in the region of the basicranial
fenestra, which it occludes in the 38-mm. stage.
T y m p a n o - m a l l e a u x . (Suborbitals.)
These are present in the 25- and 38-mm. stages. They lie
along the ventro-lateral edges of the quadrate portion of the
palatopterygoquadrate cartilages, extending from just posterior
to the articulation of the lower jaw to the level of the basipterygoid process. In the 38-mm. fish the anterior part of the
preopercular overlaps the posterior half of the suborbital. In
the 25-mm. stage the suborbitals are shown in fig. 7, PL 3
(elevation), fig. 11, PL 4 (plan), and fig. 6i, PL 3 (transverse
NO. 329
C
18
F. J. AUMONIER
section). In the 38-mm. fish they are visible in fig. 12, PI. 4
(elevation), fig. 17, PI. 7 (plan), figs. 12 i and j , PI. 5 (transverse
sections). Their relations to the cartilages are seen in figs. 26
(elevation), and 27, PL 8 (ventral view).
Lower J a w .
D e n t a i r e s . (Dentaries.)
This term includes both dentary and coronoids, for Agassiz
did not realize that the complementaries were present. The
dentary was well developed in all stages from 14 mm. onwards.
It is a stout sheet of bone which lies vertically just laterally to
Meckel's cartilage. Its lower edge is bent inward to form a
horizontal shelf which lies ventrally to Meckel's cartilage. The
dorsal edge of the dentary bears many large teeth.
Hammarberg describes the dentary as a compound bone
resulting from the fusion of a true dentary with three lateralline bones. My observations are in agreement with this, except
that only one lateral line component could be found. In two
other places mesenchyme condensations similar to those
described by Hammarberg were found, but in them boneformation took the form of an outgrowth from the dentary and
not an independent spicule.
In the 19-mm. stage the dentary is seen in figs. 1 (elevation),
3 (plan), and 1 a and b, PL 1 (transverse sections). For the
25-mm. stage the figs, are 7 and 10, PL 3 (elevation), fig. 11,
PL 4 (plan), figs. 6 a-e inclusive, PL 2 (transverse sections).
The coronoids develop later: the posterior coronoid alone
was present in the 25-mm. stage, while a small anterior coronoid
was present in the 38-mm. fish. The coronoids are immediately
dorsal to Meckel's cartilage and carry numerous teeth. They
are shown in the same figures as the dentary.
A n g u l a i r e . (Angular and articular.)
This again includes two bones, Mayhew's angular and dermarticular. The latter is just recognizable in the 17-mm. specimen
and in the 19-mm. stage has reached a considerable size
(figs. 1 (elevation), 3 (plan), 1 b and c, PL 1 (transverse sections)).
SKULL OF LEPIDOSTBUS
19
In the 25-mm. fish it projects posteriorly to the dentary,
figs. 7, 10, PI. 3 (elevation), fig. 11, PL 4 (plan), figs. &d and
e, PI. 2 (transverse sections). The dermartioular lies just
laterally to the posterior end of Meckel's cartilage, and its long
anterior end lies Between Meckel's cartilage and the ventral
horizontal shelf of the dentary. These features are more
pronounced at 38 mm. For the latter stage see fig. 12, PL 4
(elevation), fig. 19, PL 7 (plan), figs. 12 e and / , PL a (transverse sections).
Mayhew's angular first appears in the 25-mm. fish, and is
slightly larger at 38 mm. (figs, as above). It is closely applied
to the extreme postero-ventral cornu of Meckel's cartilage.
It has no relation to the lateral-line system and in the 11-cm.
stage fuses with a cartilage bone.
O p e r c u l a i r e . (Prearticular.)
This bone was only seen in the 38-mm. fish; it is a large thin
flake on the lingual side of the posterior end of Meckel's cartilage
(figs. 18, 19, PL 7; fig. 12 g, PL 5).
DISCUSSION.
When an attempt is made to arrive at a reasonable nomenclature for these bones, many difficulties arise. In giving the
present writer's views as to the correct equivalents for the
French terms used in the foregoing descriptions, it will probably
be best to deal with the bones in groups, as suggested above.
BONES RELATED TO THE SUPB A-ORBITAL LlNE.
The most anterior of the bones of the supra-orbital line, are
the median dorsal pair of the ' Os mobiles du nez'. These are
the nasals. They are much reduced and first appear immediately
underlying the most mesial pair of organs of the supra-orbital
line. They are best shown in fig. 8, PL 3, which is a dorsal view
of the 25-mm. fish: the roofing bones are seen on the fish's
right and the palatal bones on the left. (It should be stated that
the bone and its associated organ are more anterior on the
animal's left side than on the right, therefore in fig. 6, PL 2
and fig. 7, PL 3 section ' a' passes through the organ, while in
20
F. J. AUMONIEB
fig. 8, PL 3 section ' a ' passes anterior to the bone.) Unlike
the nasal of A mi a (Pehrson, 1922), this bone has a single
origin; a compound name is not therefore justified for the nasal
of L e p i d o s t e u s .
Hammarberg states that the nasal of L e p i d o s t e u s is
developed from three distinct rudiments, one of which is paired.
The whole development of this bone is stated to be the same as
that of the nasal of A mi a (described by Pehrson). In L e p i d o s t e u s o s s e u s , however, it is most obvious that no such
process occurs. The nasal is a single bone, from the earliest to
the latest stage examined. It is probable, therefore, that the
different species of L e p i d o s t e u s have different modes of
development. The pairing of the bone rudiments is also absent
in L e p i d o s t e u s o s s e u s . It is true that individual sections
of a series can be found which show such a pairing, but in all
cases the division is so short as to be insignificant and may not
be similar on the opposite side of the fish. This pairing is almost
certainly nothing more than the provision of nerve foramina.
Hammarberg thinks that the pairing of the rudiments of the
nasals is due (in L e p i d o s t e u s p l a t y s t o m u s ) to the fact
that the bone ridges which later form the tube of the lateralline canal, are laid down in the position where the two members
of the pair first appear: ' On that account it seems to me quite
natural that during development those parts of it first appear
which are thickest in the finished bone.' In L e p i d o s t e u s
o s s e u s , however, the ridges appear much later and hence
this argument is not valid.
In the 38-mm. stage the bone has increased in size so as to
cover the greater part of the olfactory organ, and now carries
four organs of the supra-orbital line.
The next bones in the supra-orbital series are those labelled
'nasaux'. They are large bones lying one on each side of the
rostral cartilage. These bones in the adult carry a considerable
portion of the supra-orbital canals; but in the young stages it
is obvious that this association is a secondary condition. The
antero-ventral extremities of the 'nasaux' carry teeth, and there
can be little doubt that they are the premaxillaries. They show
no trace of a compound origin at any stage.
SKULL OF LEPIDOSTEUS
21
Hammarberg states that the premaxillary is a compound
bone which consists of a premaxillary and two nasals. In my
specimens no such arrangement can be justified. Hammarberg
found a mesenchyme rudiment extending from the ventrolateral edge of the premaxillary to the lateral-line canal just
posterior to the olfactory organ. This condensation was also
found in my stages, but in the 15-mm. fish, although it was
present on the edge of the premaxillary, it failed to reach the
lateral-line organs. Furthermore, in most stages it was denser
at the ventral end than at the dorsal end. For these reasons
I maintain firstly, that this condensation begins from the premaxillary and is secondarily associated with the lateral line,
and secondly, that mesenchyme condensations are not a very
safe guide unless supported by other evidence. In no case were
the independent centres mentioned by Hammarberg found to
be separated when actual bone was present. It is probable that
the explanation of this extraordinary development of the premaxillaries is mechanical necessity. The snout needs support
against the stresses set up by biting, which are in a dorsoventral plane. A thin sheet of bone placed on edge would meet
the requirements of these stresses. As the premaxillary has
a posterior process in E u g n a t h u s , A m i a , and L e p i d o t u s, it is not surprising that this process has been extended
in L e p i d o s t e u s in such a fashion as to stiffen the anterior
part of the snout. The peculiar transverse curve of the posterior
process enables it to withstand considerable lateral stress, and
also to lie closely against the rostral cartilage. Since this bone
extends to the dorsal surface at an early stage, there is no
necessity for the nasal to extend posteriorly, and the premaxillary finally comes to enclose the lateral-line canal, but only at
a later stage than any of the specimens examined. This
example of a secondary association between a bone and a
lateral-line canal is interesting, but it is probable that such a
condition is rare and only takes place under conditions of
mechanical necessity.
Posteriorly to the premaxillaries come the frontals. Pehrson
describes the frontals of Amia, and says that they develop in
three parts each of which is in turn of a double origin. In
22
F. J. AUMONIEE
L e p i d o s t e u s there is no evidence of the three main components; and clear evidence against the longitudinal division
of the bones. Pehrson maintains that osteoblast rudiments are
laid down on either side of the organs of the lateral line canal,
and that the bones are thus laid down in two lateral parts. In
L e p i d o s t e u s , on the other hand, the osteoblast rudiments
are always directly below the sense organs, and the bone thus
has a single origin. In the 19-mm. stage the anterior ends of
the frontals are forked as Pehrson described them in A m i a,
and this lends a clue to the explanation of the differences in
development. In A mi a the supra-orbital line is canalized at
an early stage, while the organs lie at the bottom of a fairly
deep groove before bone-formation starts. This leaves little space
between the bottom of the groove and the chondrocranium,
hence the mesenchyme condensation is cut in two and forced
laterally. In L e p i d o s t e u s , on the other hand, the sense
organs remain at the general surface level until bone development has begun, and the grooves are only just becoming deep
at 19 mm. Therefore there is no mechanical interference with
the normal course of events, and the bones are laid down in
single shallow gutters; when, however, the groove deepens and
sets up pressure, the developing anterior ends become secondarily cleft for a short time.
Hammarberg has worked out the development of the frontal
in great detail. He describes the stages in the canalization of
the supra-orbital line, and the details of the secondary lamellae
developed from the main bone in this region. He goes on to
suggest that the frontal of L e p i d o s t e u s consists of three
elements fused. The present author cannot agree to this suggestion. The observations of Hammarberg have been confirmed, but it does not seem reasonable to use the behaviour
of secondary elements as evidence of the origin of the main bone.
It seems, therefore, that the 'frontaux principaux' are the
frontals and again that a compound name is not justified in
Lepidosteus osseus.
OTHER ROOFING BONES.
The 'parietaux' follow the frontals. They were not observed
SKULL OF L E P I D O S T B U S
23
in the 25-mm. fish, but were far advanced at 38 mm. There is
very little to be said about them; the term 'parietal' is applied
to them by all authors, and is not in questionIn addition to the above-mentioned bones there is a transverse series related to the occipital commissure, and two pairs
connected with the shoulder girdle. These are all visible in the
adult, but only those connected with the girdle were seen in the
reconstructed specimens.
The 19-mm. fish had the cleithrum and supra-cleithrum well
developed, while the 38-mm. specimen had a small scalebone
also.
However, in the 11-cm. stage all the known elements were
present in this region.
When the adult is studied, it becomes doubtful as to whether
a post-temporal exists. Immediately posterior to the supratemporal is the extra scapular which receives the lateral-line
canal from the supra-temporal, gives off the occipital commissure and finally passes the canal on to the supra-cleithrum;
from the supra-cleithrum the canal passes to the first of the
body scales. There is a small scale-bone lying postero-mesial
to the extra-scapular which contains a few pores of the laterosensory system, but no portion of the main canal, and this is
probably a neomorph.
BONES RELATED TO THE INFBA-OEBITAL LINE.
It has already been stated that the ethmoid commissure could
not satisfactorily be identified, but a few organs were noted in
relation to the antero-ventral pair of 'os mobiles du nez'.
There is no objection to the term rostrals for these two bones,
which are fused in the adult.
The lateral pair of ' os mobiles du nez' may be considered as
belonging to the same series as the elements of the infraorbitals. Hammarberg's term antorbital seems quite sound.
Maxillaires Superieurs.
As has been suggested above, these are certainly not the
homologues of the maxillary of Amia or L e p i d o t u s . They
are lateral-line bones and are developed as small gutters very
24
F. J. AUMONIEH
closely applied to individual organs of the infra-orbital line.
In their earliest stage (25 mm.) those teeth which happen to be
near to them have already made an attachment. As the bones
increase in size more and more teeth become attached, till
finally they assume the function of the maxillary: the main
tooth-bearing element of the upper jaw. It will be noticed that
the only portion of the latero-sensory system to be fully
canalized lies in this chain of bones. As has been mentioned
above, the most posterior of the 'os mobiles du nez' (antorbital) can be considered as a member of the series, though it
must be admitted that the canal does not extend on to it. The
two elements immediately posterior to this bone have fused
with each other, but all the rest maintain their individuality.
There is no evidence of separate tooth-bearing and lateral-line
components in the series, on the contrary, it is clear from the
sections that they are true lateral-line bones which have acquired a secondary tooth-bearing function. (My friend Dr.
Westoll has mentioned this fact in his work on Osteolepis.)
These bones can be called the infra-orbital chain, on account of
their relationship to the infra-orbital line. Further discussion
must be reserved till the next bone is dealt with.
' a , b , o.'
It will be convenient to make a departure from the systematic treatment mentioned above, and to deal with this element here (though it is not a lateral-line bone).
Agassiz applied these letters to a series of three small bones
just above the corner of the gape. He suggested that they
might be the 'maxillaires superieurs', while the chain of bones
to which he gave the latter name would then be called the 'intermaxillaires'.
Allis, in a paper on the respiratory valves of P o l y p t e r u s ,
mentioned this region of L e p i d o s t e u s . His words are: 'It
is, however, obvious that these bones (maxillary chain) of
L e p i d o s t e u s cannot be the homologues of the maxillary
bones of A mi a and T e l e o s t o m i , for, aside from the facts
that they have immovable relations to the skull, and are
traversed by a lateral canal, other and apparently true maxil-
SKULL OF IiEPIDOSTEUS
25
lary bones are found in what would seem to be their proper
places.'
This statement is in full agreement with my findings: the
bones 'a, b, c' represent the true maxillary. Only one bone
could be found in the series of fish studied. It always lay in
a fold of the skin ventral to the labial groove, while the lateralline canal passed dorsally to the groove. As already noted, it
bears a few teeth which are not functional in the adult. The
bone develops early, being first seen at 14 mm., while the infraorbitals develop late.
It is surprising that Allis should abandon the true interpretation in a later paper, yet this is so; for in his paper on the lateralline bones of fishes he adopts the view that the infra-orbitals
are of compound origin, and though he is apparently not
satisfied with the term maxillary chain (which he puts in inverted commas) yet he does not suggest an alternative.
Hammarberg in a recent paper has suggested the term
'lachrymalia' for the infra-orbital chain. However, it is scarcely
possible to regard these bones as being a fragmentation of the
single lacrimal of A mi a. They ought to be interpreted as
arising from a multiplication of the organs of the infra-orbital
line due to the elongation of the snout.
It is possible that this condition could be explained as follows:
The snout was being elongated comparatively rapidly and the
need for supplying a support for the teeth of the upper jaw
could be met either by lengthening the maxillary or by extending the circumorbital series. As the latter elements were lateralline bones, they had already a series of focal points for bone
deposition extending far forwards, consequently it would have
been easier for a small series of infra-orbitals to be developed
in connexion with the infra-orbital line, rather than for the
maxillary to be extended. If this were so, the maxillary would
have lost its function, and hence its degeneration is explained.
The bones labelled by Agassiz 'sous orbitaires' form an
integral part of the infra-orbital chain. They were not seen
until the 11-cm. animal was examined, and it is probable that
there is no clear division between the orbital and rostral portions of the chain until a late stage.
26
F. J. AUMONIEE
The next bones on the infra-orbital line are the ' mastoiidiens'.
These receive the line from the circumorbitals and are the
supra-temporals. Pehrson uses the term 'inter-temporosupra-temporal' for the corresponding bone in A m i a; out in
L e p i d o s t e u s no evidence of a double origin could be
detected, and thus the compound name is not justified.
The bones related to the lateral line and which lie posteriorly
to the supra-temp oral, have already been dealt with.
BONES BELATED TO THE OPEBCULO-MANDIBULAR LINE.
The only bone of this series which has been reconstructed,
and which is not a member of the lower jaw series, is the
'preopercule'. This being the case it will be convenient to
deal with the opercular series as a whole.
The 'preopercule' is clearly the preopercular. It carries the
operculo-mandibular line from the orbit till it curves dorsally
and joins the infra-orbital canal in the supra-temporal. Posterior to the preopercular lie the ' opercule' (dorsally) and the
' sous-opercule' (ventrally). The terms opercular and subopereular for these two bones respectively have not been
questioned.
Anterior and dorsal to the anterior ends of the preopercular
are the bones called ' tympano-malleaux'. The term 'interopercular' has generally been applied to the bone in this
position. Began, however, considers that it cannot be an interopercular as it is dorsal to the preopercular. He therefore
interchanges the terms preopercular and interopercular, regardless of the evidence of the lateral line, which he does not consider to be of sufficient weight. If, however, a comparison
between L e p i d o s t e u s and L e p i d o t u s is made, it seems
more likely that the interopercular has vanished and that the
tympano-malleaux' are really the most anterior elements of
the series of sub-orbitals of L e p i d o t u s . This condition could
be explained thus:
As the head becomes longer and narrower there is an increase
in the space behind the orbit, this is filled by the posterior
elements of the sub-orbitals (bearing the horizontal pit-line)
and by a new series of anamestic bones (the cheek plates). The
SKULL OF LEPIDOSTEUS
27
most anterior of the sub-orhitals, however, is squeezed between
the circumorbitals above and the preopercular below; as both
are lateral-line bones, they are more likely to survive than the
sub-orbital, which is first reduced in size, and then cut off
from its fellows.
Hammarberg calls this bone quadrato-jugal, chiefly because
he has observed a fusion between it and a cartilage bone in
the quadrate region of the palatopterygoquadrate. In Lepid o s t e u s o s s e u s , however, no such fusion occurs even at
11-cm. overall length.
PALATE.
The premaxillaries have already been discussed; the next
elements are the vomers. Goodrich considers that the term
prevomer is better for the anterior palatal bones of fishes, since
it is very doubtful whether the median vomer of mammals is
homologous with the paired bones of fishes.
The bones labelled 'palatins' by Agassiz are two quite distinct structures; the anterior elements being the palatines, the
posterior the ectopterygoids. Each of these elements is a single
bone from its earliest stage (14 mm.); there is, therefore, no
support for Mayhew's contention that the ectopterygoid of
L e p i d o s t e u s consists of a fusion of palatine and pterygoid
elements (palatopterygoid). That part of the bone called by
Agassiz ' pterygoide' and which Mayhew calls mesopterygoid,
is probably better called endopterygoid as is done by Began;
Goodrich also seems to prefer this latter name.
The 'sphenoi'de principal' is the parasphenoid. There is no
sign of a compound origin at any stage. In addition, there is no
indication of a fusion with any chondrocranial ossification even
in the 11-cm. stage.
Lower J a w .
The most obvious element of the lower jaw is the ' dentaire'.
This is the dentary but it is not a lateral-line bone in primitive
forms. In L e p i d o s t e u s , however, the ventral edge of the
dentary is folded inwards ventrally to Meckel's cartilage, and
acquires a secondary association Avith the lateral line. Here,
again, there is an urgent mechanical necessity to account for
28
F. J. AUMONIBE
the precocious development of a non-lateral-line bone at the
expense of the lateral-line series, and in this case it should be
mentioned that the organs of the mandibular line are slower
in development than those of the other lines, while this line
is the last to become canalized.
With regard to the fusion between the lateral-line and nonlateral-line bones in the lower jaw (this is the only instance
of such a fusion), the union is of a temporary nature. It seems
probable that in L e p i d o s t e u s p l a t y s t o m u s a dentary
is fused with a splenial, presplenial, and post-splenial; but
in L e p i d o s t e u s ossetis a dentary is fused with a splenial
while outgrowths of dentary invade the pre- and post-splenial
mesenchyme condensations before independent bone-formation
can take place.
On the dorsal surface of Meckel's cartilage lie the 'complementaires' or coronoids. There are two of these on each side, and
they show no features of particular interest. (In the 11-cm.
fish the posterior coronoid of Mayhew was present.)
The 'angulaire' really consists of two elements. The larger,
more dorsal element is a lateral-line bone, and is probably the
angular; the smaller element becomes fused with a cartilage
bone in the 11-cm. specimen and is thus the articular. This
reverses the order of nomenclature adopted by Mayhew, who
does not seem to have used the evidence supplied by the
sensory canals in his conclusions on this region. The articular
of L e p i d o s t e u s is of compound origin (dermal and endochondral).
On the lingual side of Meckel's cartilage and at the posterior
end lies the ' operculaire'. There can be little doubt that this
is the prearticular. Mayhew calls this bone splenial, but as
a true splenial is a lateral-line bone and lies on the outer side
of Meckel's cartilage, there seems to be no justification for this
term.
CONCLUSIONS.
The most important question which must be answered in
connexion with the above work is: 'How far can the evidence
supplied by the canals of the latero-sensory system be trusted
in establishing the homologies of the bones in which they lie ?'
SKULL OF LEPIDOSTEUS
29
It must be admitted that a final answer cannot be given here,
but it may be permissible to suggest the following principles:
1. That lateral-line bones do not fuse with non-lateral-line
bones, though they may fuse with fellow members of the
same series.
2. That lateral-line bones resist extinction or reduction more
successfully than non-lateral-line elements.
3. That a non-lateral-line bone seldom acquires a secondary
association with the lateral line, and this only happens under
conditions of strong mechanical necessity.
In the above work no attempt has been made to emphasize
the closeness of the relationship between certain bones and the
lateral canals, that would be superfluous; but sufficient evidence
has been gathered to show that the skull of L e p i d o s t e u s
can be explained without recourse to the suggestion of fusions
between lateral-line, and non-lateral-line, bones.
The dentary appears to be an exception to (1) above; but
in this case the following points suggest that the statement is
still generally valid. The lower jaw has the feeblest development of both cartilage and lateral-line organs of any part of
the skull. Also the fusion of the dentary with the lateral-line
bones does not seem to be permanent. That is to say in one
species of L e p i d o s t e u s three lateral-line bones become fused
with the dentary, but in another species only one element is
so fused.
From a careful study of the sections of all stages described
in this paper it seems perfectly clear that the expansion of the
dentary does forestall the formation of the pre- and postsplenials and does not fuse with them.
Concerning the nature of the relationship between lateralline canals and bones, no explanation can be offered. It may
be that the growth of the sense organs in the skin is initiated
by a factor which also stimulates the underlying mesenchyme
to rapid growth and later to bone-formation; as, however, bone
does not form under every organ, this is most improbable. It
is more likely that bone-formation is a balance between ancestry
and mechanical needs. In the individual this would result in
the formation of bone whenever the mechanical stress is greatest.
30
F. J. AUMONIER
The young L e p i d o s t e u s is anchored by its nose, and therefore has to stiffen its anterior end as swiftly as possible, hence
bone is laid down as soon as the mesenchyme is in a position to
do so. At a later date the pressure set up between the growing
sense organs and the underlying structures leads to bone
deposition. This, however, is nothing more than a suggestion,
as direct evidence is lacking.
The only remaining point of interest is that of the systematic
position of L e p i d o s t e u s . Since the classification of the
group Holostei is at the present moment receiving attention
from Eayner and others it is only possible to suggest that
L e p i d o s t e u s should be placed close to L e p i d o t u s . This
can be supported by the following points of resemblance:
1. The premaxillaries have long posterior extensions.
2. The circumorbitals are numerous and extend far forwards.
3. There are numerous sub-orbitals, the most anterior of which
reach the articular while the rest are immediately dorsal to the
pre-opercular.
SUMMARY.
1. The development of the dermal elements of the skull of
L e p i d o s t e u s osseus has been studied in a series of twelve
stages.
2. The homologies of the three small scale-bones of the snout
have been established. They are the rostral, nasals, and antorbitals.
3. The ' maxillary chain' is shown to be an extension of the
infra-orbital series, and the existence of a true maxillary is
established.
4. The extent and single origin of the premaxillary are shown,
and its unusual association with the supra-orbital basal is
discussed.
5. The correct homologies of the opercular series have been
established, the recognition of a sub-orbital has been recorded.
6. It is shown that Stensio's complex nomenclature is redundant to a description of the dermal bones of the skull of L e p i dosteus osseus.
SKULL OF L E P I D O S T E U S
31
^REFERENCES.
Agassiz, L., 1833-43.—'Recherehes surles Poissons fossiles', 11, JJeuehateL
Allis, B. P., jun., 1900.—"Premaxillary and Maxillary bones and the
Maxillary and Mandibular breathing valves of Polypterus biehir",
'Anat. Anz.', 18.
1904.—"Latero-sensory canals and related bones in fishes", 'Intern.
Monatssehrift f. Anat. u. Physiol.', 21.
Goodrich, E. S., 1930.—'Studies on the structure and development of
Vertebrates.' London.
Gregory, W. K., 1933.—"Fish Skulls", 'Trans. American Phil. Soe.', 22.
Hammarberg, Figge, 1937.—"Entwieklung des Schadels von Lepidosteus
platystomus", 'Acta Zoologica', 18.
Mayhew, R. L., 1923-4.—"Skull of Lepidosteus platostomus", 'Journ.
of Morph.', 38.
Parker, W. K., 1882.—"Development of skull of Lepidosteus osseus",
'Phil. Trans. Roy. Soe.', 173.
Pehrson, T., 1922.—"Cranial Development of Teleost fishes", 'Acta
Zoologiea', 3.
Regan, C. J., 1923.—"Skeleton of Lepidosteus with remarks on the origin
and evolution of the Lower JSTeopterygian Fishes", 'Proc. Zool. Soc'
Veit, O., 1907.—"Besonderheiten am Primordialcranium von Lepidosteus
osseus", 'Anat.', 33.
—— 1911.—"Entwieklung des Primordialcranium von Lepidosteus
osseus", ibid., 44.
EXPLANATION OF PLATES 1-8.
Allfiguresare of L e p i d o s t e u s o s s e u s . Left-side views
are 'elevations', and dorsal views are 'plans', drawn as projections without perspective.
The following symbols are employed on the figures:
Dermal bones: fine line stipple.
Cartilage: coarse dot stipple and fine dense dot stipple.
Lateral-line organs: heavy black dots.
PLATE 1.
19-mm. stage.
Fig. 1.—Left-side view showing dermal bones and lateral-line organs.
Figs. 1 a, b, c.—Transverse sections at the stations indicated by the
lines a a, &c.
Fig. 2.—Dorsal view showing dermal bones and lateral-line organs.
Fig. 3.—Dorsal view; lower jaw alone, showing dermal bones.
32
F. J. AUMONIER
Kg. 4.—Left-side view showing palatine and ectopterygoid.
Fig. 5.—Left-side view showing prevomer, ectopterygoid, and parasphenoid.
PLATE 2.
25-mm. stage.
Fig. 6.—Left-side view showing dermal bones and lateral-line organs.
Figs. 6 a, b, c, d, e, f, g.—Transverse sections at stations indicated by
lines a a, &c.
PLATE 3.
25-mm. stage.
Figs. 6 h and i.—Transverse sections at stations indicated by lines
h h, &c.
Fig. 7.—Left-side view showing palatopterygoquadrate, dermal bones,
and lateral-line organs.
Fig. 8.—Dorsal view showing dermal bones of dorsal surface and organs
of supra-orbital line (on fish's right). Dermal bones of palate and
organs of infra-orbital line (on fish's left). The parasphenoid has been
cut off at section d.
Fig. 9.—Dorsal view of anterior end (to section / ) showing palatal bones.
Fig. 10.—Left-side view of anterior end (to section / ) showing dermal
bones of palate and lower jaw.
PLATE 4.
25-mm. stage.
Fig. 11.—Dorsal view of lower jaw showing coronoids, Meckel's cartilages,
and quadrate region of palatopterygoquadrate (on fish's right); dentary,
angular, articular, and sub-orbital (on fish's left).
38-mm. stage.
Fig. 12.—Left-side view showing dermal bones and lateral-line organs.
PLATE 5.
38-mm. stage.
Figs. 12 a, b, c, d, e, f, g, h, i, j , k.—Transverse sections at the stations
indicated by the lines a a, &c.
PLATE 6.
38-mm. stage.
Fig. 13.—Left-side view showing dermal bones of infra-orbital series.
Fig. 14.—Dorsal view showing dermal bones of dorsal surface and of
the infra-orbital series.
Fig. 15.—Left-side view showing prevomer and parasphenoid.
Fig. 16.—Left-side view showing palatine and ectopterygoid.
SKULL OF LEPIDOSTBUS
33
PLATE 7.
38-mm. stage.
Fig. 17.—Dorsal view showing palatal series.
Fig. 18.—Left-side view showing coronoids, maxillary, prearticular,
and angular.
Fig. 19.—Dorsal view of lower jaw.
Figs. 20 and 21 have been placed in the text as Text-figures 1 and 2.
Fig. 22.—Dorsal view of chondrooraninm. A portion of the supra-orbital
bar on the fish's right has been removed to show the artioulation of the
palatopterygoquadrate with the basitrabecular process.
Fig. 23.—Left side of ehondrocranium.
PLATE 8.
38-mm. stage.
Fig. 24.—Ventral view of ehondroeranium. The palatopterygoquadrate
and hyomandibula have been removed on the fish's right to show the
cornu trabeeulae and basitrabecular process.
Fig. 25.—Dorsal view showing chondroeranium.
Fig. 26.—Left-side view showing chondroeranium and dermal bones.
Fig. 27.—Ventral view showing chondrooranium and dermal bones.
The anterior end of the eotopterygoid, palatopterygoquadrate, and
hyomandibula have been removed on the fish's right.
NO. 329
Supra orbital line
Frontal
commissure
Labial groove
axillary-
MeckelV
cartilage
Maxillary
a b
Dentary'
Angular
Operculo mandibuiar
line
line
a b
Premaxil!arLj%
Horizontal pit
line
Septum rostri
Palatal region of
rygoid -illary —
Roof of neurocranium
a b
,Supraorbital line
Dentary' y
Angular
W)
1b
M ,, „
Meckel's
!,
., cartilaqe
3
Parasphenoid
Palatal region
; ' of
Pdlatoptefygociuadrace
\\
Ectopterygoid>
Angular
1c
20040
Ra ra sph enoid
a b
II
Ectopterygoid
Parasphenoid
, -Middle pit
line
- Occipital
commissure
xi Mary - - 5-s^TT"
Palatine
Frontal
a b
Parasphenoid
Ectopterycjoid
Prevomer
Dentary
iupratemporal
.Frontal
Pa
-ip
N
h
Premaxillary
i
Septum rostri
',
Palatal region o f
palatopterygoquadrate
Premaxillary
^--Coronoid
Prernaxillary--
ry —
LL
6a
Dentary
Septum rostri
naxillary
',
tal v
9
L—Ma
"Pre
an
"
Coronoid
6b
Palatine
Meckel 's
cartilage
--Ec
Septum rostri .,
_ !
.Nasal
or
Bianchi
Meck
cartil
Denta'ry
Parasphenoid
Meckel's
cartilage
Palatal region of
, - 'palatopterygoquadrate
--Ectopterygoid
Septum rostri
j Olfactory lobe
Olfactory nerve
Quadrate region of
palatopterygoquadrate ~ ~-
Ectopterygo
Dentary
r
Parasp'henoid
-Vertical pi
•Meckel's c
Dentary
6d
An
9ul'a'r
Parasphenoid
6e
Neurocranium
Supraorbital
Supraorbital line
Frontal
Retina
Quadrate region of
" "palatopterygoquadrate
gion of
^
goquadrate;/
Angular
--""-"-Meckel's
cartilage
Parasphenojd
r
g
f
_ _ Opti
nerv
Basicranial
trabecula
Quadrate region o
'—Hyomandibula
Preopercular
-
Branchiostegal
ray
s
Branchiostegal ray
6h
h i
gan
n obital Maxillary
>
\
Coronoid
_
mA uian lar'
9 Articular<
'
•
\rrerugoiaregionor
N —*x
Ic Id
/a
c d
/Parasphenoid
— ^
Subdrbital *»palatopterygoquadrate X Preopercular
\ .
Branchiosfceoal
rau
Branchiosbegal ray
p Quadrate
0
Supra orbital line
PremaxJIIaryX
rgan
mer
,.Supratem
.Frontal
le |F
|h 1% _
Frontal
Prevomer
200400M
wCoronoid
Dentaru \l
felatine, \
Bran
I^Vi
Anqular
fa
Ib
peckers cartilage
Coronoid
Coronoid
0 200 400JUL
ab
c d e
Articular
f
g
h i
^Suborbftaf
J k
/Parietal
Supratemporal
..-"'
,,-"Scalebone"
Frontal
Premaxillary
ab
c d
--Supracleithrum
--Opercular
Coronoid
if^O
12 a
Dentary
12e
'cartilage *
Rostral
Septum rostri
Septum rostri
!
N
Nasal
Olfactory
organ '"
Septum rostri
.Olfactory
Premaxillary-. ;
Infraorbital
canal
Premaxillary
^Infraorbital
Prevomer
Oentary
•, ^ v# v , ^-Infraorbital
Ectopterygoid - - ^ ^ " ^ - - P a l a t i n e
12 f
Prevomer \ $ ^ 4 ~ = ^ u
;/
.
Dentary
Meckel's Parasphenoid
cartilage
•—•—>
0 400 800 JJL
Septum rostri
Premaxillary v l _ , F r o n t a l 1 Z 9
Palatal region o f ....
P 5
l t p ty gd q t e ~
Ectopterygoid ^
Palatine
Prevomer
D e n t a r y ' / > i l ^ ^ t ^ " Meckel's
Angular'
. . Septum rostri
Frontal
< \
ParietaI
Palatal region of
palatopterygoquadrate
region o f ,.
-Dentary
ygoquadrate
x "Angular
Articular
. , .
Ectopterygoid /
Suborbital
Prearticular Parasphenoid
12 h
Ectopterygoi'd
tal
- - - — •
FrontaJ
Parasphenoid
f
Parietal
Parasphenoid
•
|
,Frontal
. Basicranial
trabecula
,-.NPterygoid reg
vv palatopterygo
Preopercular
Parasphenoid
0 400 800
Posterior taenia
tecta media
SupratemporaJ
121
^
Basicranial
trabecula "
/.^Quadrate region o f
palatopterygoquadrate
Ectopterygoid
cartilage
Roof of neurocranium
— ^_^ -,
Preopercular
Paras ' phenoid
Frontal
ySupraorbital bar
Pterygoid reg
i "palatopterygo
~—Ectopteruqo
v
Hyomandibu
1 2 k
Organ of infraorbital line
\Pore of inftaoitital line
sed portion of \x ! '. infraorbital
aoroital line
• —
^Maxillary
^ ^
I
13
^C
al y ^ a s a f ^
Maxillary
.Premaxillary
Anforbibai
j Anterior nostril
actory organ
L
0 400 800JJ
Supratemporal
Sc
N
\ Antorbital
Anterior nostril
\ \ \
c
A.
d
e
e
/ I
infraorbital
Frontal
g
H
9
h i
Parietal'
Supracleithrurh-
'i
j
//Rarasphenoid
Prevomer
0 400 800|x
mer
Palatine
17
Suborbital
9
Coronoid
d
ii ik
h'
IP
,Maxillary
Dentary
*
—
— -
BranchiostegaI
q
h
I
I
Angular
(
^ _ ^ _ _ ^ _ _ Coronoid ,
Anqular v n \ ,
'
3
Particular
Coronoid''
18
19
If
Id fe
<z
i
c d
i i
9
j
alatal region of
h
I
palatopterygoc|uadrate
j
i
D
\.
,
A
Prearhcular
Ig
Ih
J
i
| j
Basitrabecular process
.Quadrate region of /,Basicranial trabecula
•Dalatooteruoofluadmte//
j^teriorbenia tect
region
pterygo
22
Cornu brabeculae
m rostri
\
Posterior fontaneile
Anterior fontanelle
Supraorbital bar - - .
ta
tecfca me
23
x
s
D M region cF pala^rygoquadrate
->-
v
'"Basitrabecular proc
Quadrate region of palatopterygoquadrate
^- palatopterygoq
al region of
oDteruooauadrate
Posterior
tecta m
',
Basicra
"trabecu
ptum
stri
cornu M t c u t a
Supraorbital bar
•Basitrabecula
Posterior taenia tecta media P r o c e s s
Palatopterygocfuadrate /supratemporal /Scalebo
Premaxilla
sal
Supra
^cleithr
Cornu trabeculae
H I M
ab
dl
o
a b
/! I
I
e.
25
II
j
F
c d
l
c
h
I
l
Palatal region of
palatopterygoquadrate
Premaxilla
Nasal
epfcum
ostri-
Kostral
Frontal Anteri(/r f o n taneIleX a r s e t a l Posterior fontanell
IS
\
I I Basrcrania/ trabeculs
Basicranial
I I trabecula
b l
Parasphenoid /
,
Frontal
/
/
/
^Suprate
\s^Subopercu
y
Suborbital .
.
Endopterygoid \
\
'Preope
\Basitrabecular pro
Quadrate region of
palatopterygoquadrate
Pterygoid region o f
palatopterygoquadrate
Ectopterygoid
| _ ^ — ^ _ _
___
- -Ope
Hijoma
Ectopterygoid
s
Prevdmer
26
Prevomer
ostral
!
;
pbum rostri
Pa | at f ne
Parasphenoid
|
Endopterygoid \freopercular \Subope
Suborbital
Basitrabecular proce