374 MR. G. S. WEST ON 
variation in the form of the cell, and although in the majority of 
species the vertical view is more constant in outline than the 
front view, yet there are some (e. g. Cosmarium biretum, Bréb.) 
in which it is very variable. 
With regard to the ornamentation embellishing the exterior 
of the cell-membrane, much more detailed observation is required 
before any precise statement can be made eoneerning its 
variation. In some species the variation is in the arrangement 
of the warts, granules, scrobiculations, spines, &e., with which 
they are aderned ; and in these cases there may be a considerable 
range of variation, which nevertheless does not, except under 
rare circumstances, transgress those laws which regulate the 
disposition of the adornments on any particular species under 
consideration. A very good example of this is seen in Cosmarium 
Fig. 2. 
atot D WË D 
Dee ^ ki MA so SUN, 
o e? ae 9$ [J o oo ep D M 
96 x De Ze PA d oo o KA L 
D ° 
099 b so goo e g 
a 
. o LT 
m LEA ejt ea 
aSo Die? NW Do? 2 "3 
De So ei one DEA . . o 
0 0 DO 0% *. "a "rege Se, ef 
[e] LJ D LJ LJ 
oo k 
c f ] 
Central serobieulations of Yanthidium antilopeum, Kuetz. x 520. 
a-d, from Thursley Common, Surrey ; e, from Ballynahinch, W. Ireland ; 
J-k, from Pilmoor, near Thirsk, N. Yorks. 
orthostichum, Lund.*, in which the large granules ornamenting 
the species are arranged ona fairly consistent plan and yet them- 
selves exhibit a notable variation, especially in the centre of the 
semicells (cfr. Pl. 11.figs. 1—4). Another illustration of this 
arrangement is afforded by Staurastrum vestitum, Ralfs, in which 
the emarginate warts on the dorsal surface aro always disposed 
on the same plan, no matter how variable the inferior series 
(including the two furcate spines) may be. The central serobicu- 
lations of Xanthidium antilopeum, Kuetz., although showing 
much variation with regard to their size and the details of their 
disposition, are also arranged approximately in the form of a 
ring (fig. 2, a-k). 
* P. M. Lundell, “ Desm. Suec.," Act. Reg. Soc. Scient. Upsal. 1870, p. 24, 
t. 2. f. 9. 
