258
BRITISH DESMIDIACEJD.
truncate, sides straight and 4-crenate, crenations
bigranulate (except the basal one or two), apex dilated
and 4-crenate, apical angles (formed by the two lateral
of the 4 crenations) prominent and rounded, crenation
bigranulate (apical angles often 4-granulate) ; within
the margins with radiating series of granules disposed
in pairs within each crenation; in the centre of the
semicell and towards the isthmus with a broad tumour
furnished with 4-7 (commonly G) vertical granulate
ridges (3-5 granules on each ridge). Side view of
semicell rectangular, slightly tumid near the base at
each side, sides slightly retuse, apical angles rounded
and granulate. Vertical view elliptic, with a broad
crenulate tumour on each side, poles granulate.
Zygospore unknown.
Length 45-50 JU, ; breadth 34 8-40 /A ; breadth of
isthmus 18-20 /A ; thickness 23-4-28 ^ .
ENGLAND.—Mickle Fell, N. Yorks !
"WAI.ES.—Near Bethesda, Carnarvonshire !
SCOTLAND.—Black Isle, Ross; Koynach Moor, Presswhin, Craigendinnie, and Aboyne, Aberdeen; Den of
Grarrol and Dunis, Kincardine (Roy
Bissett).
Glen
Shee, Perth!
IRELAND.—Dublin and Wicklow (Archer).
Geogr. Distribution.—Norway.
Russian Lapland.
N. Russia. Nova Zembla. Spitzbergen. Greenland.
-
This alpine and arctic Desmid is exceedingly rare in the
British Islands. It occurs principally in boggy springs and
other small boggy areas high up on the mountains. It appears
to us to be a very distinctive species, but it should be very
carefully compared with C. costatum, a Desmid which occurs
in similar habitats.
W e figure a small form of C. hexalobum from Penyghent,
W . Yorkshire, which differs only in its dimensions and is
therefore not to be included in var. minus Roy & Biss. ; length
36JU ; breadth 33/x; breadth of isthmus 18/*. (PI. XO, fig. 6.)
Var.
minus
Roy & Biss.
(PI. X C , fig. 5.)
C. hexalobum Nordst. var. minus B o y & Biss. Scott. Desm. 1894, p. 103,
t. 1, f. 1 2 ; W e s t & G. S. W e s t , Alg. S. England, 1897, p. 488; Gutw. in
Spraw. Kom. fizyog. Akad. Umiej. Krakow. 1898, p. 152.