WALKER-ARNOTT, ON ARACHNOlDISCtJS.
159
arm has been carried up to the highest point of the vertical
stem, and the tody of the microscope placed vertically, it
may be used on the surface of the fluid of an aquarium
standing two feet high; the upward and downward movements of the transverse arm being then used for fine adjustment.
This arrangement was made for me by Messrs. Farquhar
and Gill, plumbers and gas-fitters, of this city, for less than
thirty shillings; but it must be borne in mind, that for cheapness it is essential that selected pieces of ordinary brass
tubing be made use of, that the vertical stem and the transverse arm be pieces of the same tube, and the pieces, g and
h, parts of another tube; also, that the ball-and-socket joint
be the one ordinarily used by gas-fitters. Many of the details of the arrangement, especially those connected with the
various pinching screws, may appear tedious in description,
but it will be found that these are points of great consequence for securing comfort and facility in making any observation, and therefore I have described them at length. The
application of a good rack-and-pinion for the movement of
the body of the microscope would be a valuable addition, but
it would increase the expense considerably.
On ARACHNOIDISCTTS.
By G. WALKER-ARNOTT, LL.D.
Before having a complete knowledge of the natural history
of Diatoms, it is necessary that we know—1st, where, when,
and by whom any object was first observed and brought
under the notice of naturalists, whether by name, description, or a figure: 2d, where, when, and by whom it was first
correctly named and defined by a precise differential (generic
or specific) character, ,-the latter of these alone giving a
claim of priority.
/
It is not twenty years' since the genus Arachnoidiscus was
known. Short as that period is, I have not been able to trace
its history with satisfaction. I shall, however, indicate here
the information I have obtained, and hope that those connected
with its discovery and description will complete the sketch
before it be too late: already one (Dr. Bailey) has been
removed.
It is generally said, and I believe with justice, that
160
WALKEH.-AHNOTT, ON AKAOHNO1DISCU§.
Mr. Topping, and others in London, first observed these
disks in Ichaboe guano. This guano was discovered in 1843,
and was nearly all removed in the course of 1844.* Although
guano was long known on the west coast of South America,
I do not find that it was noticed for its diatoms until after
that of Ichaboe was examined.
Ehrenberg describes this genus under the name of Hemiptychus ornatus: when and where that name was given I have
not traced, but believe it is in the ' Berl. Acad. Trans.' for
1848 or 1849. The description given in Pritchard's 'Infos./
2d. ed., p. 382, shows that it is the form with transverse
costse, and it is said to have occurred in " Patagonian
guano." • But here let me state that there is great difficulty
in tracing the origin of guanos, not only from their being
adulterated or mixed by the guano merchants, but by the
preparers of objects for the microscope mixing what they got
from different ships, under the impression that they were
brought from the same place. Thus, some years ago, I purchased a slide of Diatoms from guano understood to be from
Africa (Ichaboe); this contains the usual blue discs of that
guano, but besides these is a valve (broken by pressing down
the cover) of Eupodiscus (Aulacodiscus) formosus,-f which is
peculiar to Bolivian guano. Here some guano from Arica had
been mixed with that from Africa, the similarity of name probably leading to the supposition that the two samples had come
from the same locality. In Ehrenberg's ' Mikrog./ tab. 35,
he gives a representation of " Peruvian" guano. In the
description of the plate, however, it is stated to be from
Arica, which is in Bolivia or Upper Peru, not in Peru as now
limited. In the same work he exhibits the diatoms of two
samples of guano from Saldanha Bay. The sample A. appears correctly designed; but in B.. all the species noticed
(Endyctia oceanica, &c.) are so copious in Peruvian guano
(called also Callao or Chinca), that I have no hesitation in
saying that Ehrenberg must have misplaced the labels of the
samples.
I therefore doubt if the Hemiptychus ornatus was derived
from Patagonian guano; but as the same form does occur in
Californian guano, I dare not say that Ehrenberg's was not
from South America.
* Ehrenberg's earliest notice of guano diatoms was in 1814, and liis
sample appears to have been obtained from London, and probably was
derived from lelmboe.
f Eu. (A)formosus cnpulis quatuor submarginalibus oblique niaimiKcformibus apicc papillo instructs, granulis in 1-1000 parte septem (sive in
1-100'" parte sex) subajqualibus.
WALKISR-AHNOTT, OK AllACUN.OIDISCUS.
It31
In Smith's ' Synopsis of British Diatomacese/ p. 25, the
genus Arachnoidiscus is said to have been proposed by Bailey.
But I have before me the following extract of a letter from
Dr. Bailey, of date July 27th, 1853:—" I see that Smith, in
his ' Brit. Diat./ gives me as the founder of the genus. This
is not correct, but the species is mine, and it is very different
from the A. Japonicus with which Smith confounds it." The
founder of the genus was Mr. H. Deane, of Clapham, and it
was first noticed in a paper read by him before the Microscopical Society on 17th March, 1847. This paper was not
published, and although it contained a general description of
the disk, no distinguishing character was given. Mr. Shadbolt, on 14th November, 1849, read a paper "On the Structure of the Siliceous Lorica of the genus Arachnoidiscus,"
and confirmed the generic appellation. In Pritchard's ' Infusoria/ 2d. ed. (1852), the generic character will be found,
and there also the name is correctly ascribed to Mr. Deane.
In the ' Micrographical Dictionary' it is said that Ehrenberg
had now withdrawn the name Hemiptychus, as there was
already a Hemipticha, a genus of Hemipterous insects.
I now come to the .species. In Pritchard's 'Infusoria/
page 700, the species there figured is called A. Japonicus of
Shadbolt. Now Shadbolt's specimens (figured in the ' Micr.
Soc. Trans./ iii.) were from South Africa, and (if there be
really more than one species) are not the same as the Japan
form, and consequently not entitled to that name. Then
again Bailey, as already said, gave the name of A. Ehren*
bergii to a species from California (Puget Sound), which he
supposed to be quite distinct from "A. Japonicus." I cannot
find that Bailey ever published this species; but Smith, in
bis ' Brit. Diat./ adopted it on the authority of De Brebisson,
quoting A. Japonicus of Pritchard as a synonym. It is not
very clear to me which Dr. Bailey meant. I have examined
a slide prepared in 1853 from the Puget Sound form (got off
an alga), and find it identical with the Japan one, but not
with what is figured by Shadbolt or Pritchard; and another
prepared by the late Professor Smith, and marked by him as
obtained by Professor Bailey from California, and sent on
22d October, 1856: but this is the African form figured by
Shadbolt; so that, if there be no mistake on the part of Dr.
Bailey or Professor Smith, Dr. Bailey at first called the
Japan form A. Ehrenbergii, and afterwards applied that name
to the "A. Japonicus, Shadb.," or African form. Smith has
certainly not shown his usual sagacity in the elucidation of
this genus; his generic character is nearly the same as Shadbolt's and Pritchard's, but does not apply to the figure given
162
WALKER-ARNOTT, ON ARACHNOIDISCUS-
in his plate 31. I can only explain this by supposing that
Mr. Tuffen West, in making the drawing had employed a
specimen of the true Japan form, perhaps from Mr. Deane;
•while Smith had derived his generic character solely from
African specimens, aided, perhaps, by Mr. Shadbolt's figure,
which he praises. In the African form there are irregular
costse or lines connecting the radiating lines, in addition to the
granules, and the granules are small; in the Japan form the
granules are large, and placed in transverse rows, but there
are no transverse costae. A slight comparison of Smith's
figure with Shadbolt's or Pritchard's will make this difference
obvious.
In the ' Mikro-geologie,' Ehrenberg figures two species,
both from earth, from the Island of Camorta, one of the
Nicobar group. His A. Indicus is quite the same as the
Japan one, having no transverse costse, while the A. Nicobaricus seems the same as the African form.
If there be two distinct species, as is probable, the one
may be called A. Ehrenbergii, to comprehend the Japan
species, and that obtained by Dr. Bailey from California
prior to 1853, as also A. Indicus of Ehrenberg: the other, A.
formosus, to contain Hemiptychus formosus, Ehr., A. Japonicus, Shadb., and A. Nicobaricus,lEih.v. I have already pointed
out how these are easily distinguished. For A. Ehrenbergii
I can only indicate the Japan seas, California, and the
Nicobar islands, as the localities whence obtained. For A.
formostis may be assigned a much wider range, as South
Africa, Nicobar Islands, and West Coast of America. Which
the British one is I cannot say; I fear there is a mistake
about its occurring in our seas.
It is not improbable that of A. formosus there are several
varieties; in some, 1 find the transverse costse quite simple,
in others, much and irregularly branched, like the veins of
the leaf of a dicotyledonous plant, and in a form which I
have from Mauritius (growing upon Plocamium Telfairiee), the
radiating costae frequently (if not always with a good light)
pass between the double row of puncta around the pseudonodule, and reach the pseudo-nodule itself; this structure
requires to be verified from other localities; it seems intended
by Ehrenberg in his figure of A. Nicobaricus.
In all that I have examined, taken from off the Alga, the
lower valve is thinner, and sometimes differently marked
from the upper one; the characters I have indicated are
taken from the upper valve only.
There can be no doubt that these discs have (as said by
Shadbolt) a horny vegetable outer covering in addition to
WALKER-ARNOTT, ON ARACHNOIDISCUS.
163
the siliceous one, and that by too long boiling in acid, as is
necessary for guano, the marks are much obliterated, or entirely removed. This, however, is not peculiar to the present
genus, but may be observed, more or less, in all diatoms,
although sometimes the vegetable pellicle is very thin, and
may be removed by a few seconds' immersion in boiling nitric
acid. It is this circumstance which gives a quite different
appearance to the same species, according as the preparation
is made. Thus, in Actinocyclus the vegetable epidermis is
cellular, while the siliceous part is striated like a Pleurosigma;
and when the vegetable part is removed, we often find nodules
or knobs along the margin (forming, then, the genus Omphalopelta), not previously visible. Those who describe diatoms
from slides are thus liable to commit great errors, and indeed
no certainty can be obtained, except by getting the recent or
growing diatom and examining it—1st, after being immersed for a short time in cold acid, or simply washed in
boiling water; 2dly, after being boiled in acid for about half
a minute, or a whole minute at most; and 3dly, after being
boiled for a considerable time. We shall then see that many
of the supposed distinct species of authors are the same, prepared in a different way. Of course deposits or guanos can
yield little or no information; although once a species has
been determined by the way I have indicated, we may be
able to refer forms occurring in guano or deposits to it, with
tolerable certainty.
In my paper on Rhabdonema, in the last number of this
Journal, I described the genus Eupleuria .• since then I have
found E.pulchetta, not uncommon on Ballia, from Cape Northumberland, in South Australia. In that paper I noticed that
E. incurvata differed from the others by the annuli not beingcellular ; it is therefore probable, that it will have to be removed from the genus, particularly if the supposed annuli in
that species prove to be only the siliceous connecting zone
split, as it occasionally does in various other genera, into thin
lamina. That this may be its true structure is rendered possible by the discovery of a new genus from Mauritius, growing
on Plocamium Telfairice (along with the Arachnoidiscus). This
new genus has certainly no annuli: the upper and lower
valves are as described in Eupleuria, and consequently it is
intermediate between that genus and Achnanthes; differing
from this last by the want of a stauros to the lower valve;
by the costae not proceeding to the extremities, at least, on
the lower valve; and by the valves being merely arched,
and not geniculate; it has no stipes, and seems attached by
the side, as in Eupleuria. To this genus the name Gephyria
164
WALKER-ARNOTT, ON ARACHN.OIPISUUS.
may be given, the more especially that Eupkuria incurvata
(my original Gephyria) may be removed here. The costje
are about 6 in '001 in the Ichaboe species, while in the
Mauritius one they are much closer, 15 in "001. This last
may be called Gephyria Telfairice, after the late Mrs. Telfair,
who discovered the Alga in which it occurs. In this the extremities of the frustules are sharp; but I have, apparently,
the same species from the West Coast of Australia (obtained
by washing some Algte collected by Mrs. Drummond, and
sent me by Dr. Harvey), but in that the frustules are more
obtuse.
In my former paper I described Amphiprora Ralfsii; in the
same number (' Trans.') is a paper by Dr. Donkin, to which
I find it necessary to allude, on account of the want of
courtesy there shown (p. 33). When I transmitted my notes
to Professor Smith, Mr. Ralfs, or others, they were to be
held as mere notes; and although any gentleman is at liberty
to see them, or to use them, after verifying them,, all that I do
not myself publish must be regarded as private communications, and with which my name is not to be connected, if
published by others. Dr. Donkin gets some information
from Mr. Roper, and Mr. Roper gets his from Dr. Montgomery, and Dr. Montgomery gets his from Mr. Ralfs, who
gave a slide, with a name attached, which name I have now
published. But I beg to say that Mr. Ralfs' slide was not
from material discovered by me, as Dr. Donkin asserts, and
that the identification of Dr. Donkin's Pleurosigma rectum
with my Amphiprora Ralfsii, was not made by me. If Dr.
Donkin wishes to know what my species is, he must not
go to a slide named by others, or by myself, containing
several objects, but to my specific character,* for in drawing
it up I had reference to several forms, both in Mr. Ralfs'
gathering and found elsewhere; and any one may see from
it that several of Dr. Donkin's supposititious species are
combined under that character; in fact, Mr. Ralfs' gathering
contained, so far as I can comprehend his descriptions and
figures, Pleur. rectum, Wansbeckii, minutum, and probably
also angustum, which I consider one and the same species of
Amphiprora. Pleur. carinatum I ought perhaps to add to the
list, for I believe that the strife only appear oblique in consequence of the position of the light; if a true Pleurosigma it
may be Pleur. obscurum, the only one with that peculiar ap* I might have made tlie diagnosis more precise by saying I lie valves,
although caviuate, are not ululu. This, however, is.implied by not.-noticing
WALKER-ARNOTT, ON A li ACHNO[DISCUS.
165
pearance. Dr. Donkin will allow me also to add, that his
S. V. and F. V. of Pleur. lanceolatum, belong not only to different species, but to distinct genera; that his Pleur. arcuatum
is only Pleur. fasciola: his Toxonidea insignis, the wellknown distorted state of Pleur. cestuarii; his Tox. Gregoriana
the same of either Pleur. strigosum, or angulatum (I have
seen both distorted), but which I cannot say from his imperfect diagnosis of the species. His Cocconeis excentrica was
discovered by De Brebisson, in 1852, and was then called by
him C. orbicularis; and his Epithemia marina, the E. Radula
of the same French gentleman, afterwards distributed in slides
by Professor Smith as Nitzschia Radula ; this I have long had
from the Clyde, and also Teignmouth. His Arnphiprora
duplex, judging from the figure and diagnosis, scarcely differs
from A. paludosa. It is to be regretted that Dr. Donkin did
not make himself acquainted with what others are doing,
before rushing into type; and that, instead of giving the long
descriptions and figures, and multiplying species unneces-,
sarily, he had limited his species by a short, concise character,
as every true naturalist must do who wishes his species to be
adopted or considered by others.