VARIATION IN THE DESMIDIEA. 389 
In 1894 Eichler & Gutwinski deseribed a var. polonicum of 
Cosmarium Nove-Semlie, Wille (Rospr. Wydz. matem.-przyr. 
Akad. Umiej. Krakow. tom. xxviii. p. 170, t. 5. f. 27), and the year 
following Sehmidle described a var. montanum of C. Regnesii 
(cfr. * Hedwigia, 1895, p. 74, t. 1. f. 9; Oesterr. Bot. Zeitschr. 
xlv. 1895, p. 389, t. 15. f. 11). About the same time a species was 
described as C, Pseudoregnesü (cfr. West & G. S. West, Trans. 
Linn. Soc., Bot. ser. ii. vol. v. p. 59, t. 6. ff. 42-43) ; and shortly 
afterwards these were shown to be identical forms, all referable 
to C. Regnesii or C. Pseudoregnesii (cfr. West A G. S. West 
in Journ. Bot. xxxiv. 1896, p. 336-7). Schmidle has since termed 
his form C. montanum ` but I think that a consideration of the 
variations deseribed above as occurring in C. Regnesii proves 
conelusively that his species cannot be separated from the latter 
except as a variety, and therefore all the above-mentioned 
forms will fall under C. Regnesii, Reinsch, and its var. montanum, 
Schmidle. 
10. CosMarium BIRETUM, Bréb., in Ralfs, Brit. Desm. 1848, 
p. 102, t. 16. f. 5.— This Desmid is by no means frequent, 
and seems to have a preference for the marshes of low-lying 
districts, in which situations it is sometimes obtained in 
abundance. 
Ina gathering of an immense quantity of this species from 
Welsh Harp, Middlesex, many variations in form were observed. 
The typical form of the semicells may be described as subrect- 
angular with the lateral margins slightly divergent, the basal 
and apical angles rounded, and the apex somewhat convex. The 
divergence of the lateral margins varies very much (cfr. Pl. 10. 
figs. 22 & 26), and this causes the semicells of some forms to 
possess a much broader apex than those of others. The con- 
formation of the apex is also markedly different in different 
individuals. In some it is straight or but slightly convex, and 
may be even retuse in the middle, whereas in others it is strikingly 
elevated, being very convex and often truneate in the median 
part (Pl. 10. figs. 25, 26). These two conformations of the apex 
may, however, be found in the semicells of the same individual. 
There is a considerable range in the size and in the roundness 
of the angles of this type of C. biretum, distinguished by its 
single inflation on each side of the vertical view. This latter 
charaeter varied, in specimens from the same gathering, from a 
