VARIATION IN THE DESMIDIER. 391 
contour and rarely to possess semicells with divergent lateral 
margins. The three protuberances on each side of tbe vertical 
view may be slight and more or less of equal size, or the central 
one may be much larger than the other two, the latter varying 
in their relative proximity to the ends (Pl. 10. figs. 32 6, 33, 
& 84). 
From the table on p. 390 it is seen that although the first 
type is relatively a little longer than the second, the latter 
reaches the largest dimensions, especially with regard to 
thickness. 
The granulation of these two types is precisely the same ; in 
fact, I should be justified in saying that the granulation of this 
species is its most constant feature, being eminently characteristic 
of all its forms. 
11. CosMARIUM ORTHOSTICHUM, Lund., * Desm. Suec.,’ Act. 
Soc. Scient. Upsal. 1870, p. 24, t. 2. f. 9.— This species is 
characterized by the possession of large granules arranged in 
approximately vertical and horizontal series across the surface 
of the semicells. The vertical arrangement is generally more 
readily discernible than the horizontal, the latter being at times 
replaced by oblique series. In the centre of the semicells 
the granules exhibit a variation with regard to their relative 
size, and in certain individuals one or more of them may be 
duplicated. In the vertical view the granules may show a 
distinct arrangement in transverse lines, or a clear space may 
be evident in the centre. Cfr. Pl. 11. figs. 1-4. 
19. STAURASTRUM MUTICUM, Breb., in Menegh. * Synops. 
Desm.,” Linnea, 1840, p. 228.—In a gathering of a large quantity 
of this species from Roundhay Park, W. Yorks., the semicells 
were observed to vary considerably in outward form. In some 
examples they were perfectly elliptical, in others reniform, in 
others elliptico-semicircular, and in many cases distinct basal 
angles were present, causing an almost linear sinus. Speci- 
mens were noticed with one semicell triangular and the other 
quadrangular, but the presence of both a three-angled and a 
four-angled semicell on the same plant is a variation of very 
frequent occurrence in species of this genus. 
13. STAURASTRUM BRACHIATUM, Ralfs, Brit. Desm. 1848, 
p. 131, t. 23. f. 9.—This species frequents the most suitable 
