COSMARIUM. 37 



mountainous areas it is not uncommon in boggy springs and 

 amongst mosses on dripping rocks. In more lowland districts 

 it is principally confined to ancient lieaths. 



The common British form has three crenations at each side 

 of the semicell and four at the apex. Specimens are not 

 uncommon, however, with four crenations at each side, the 

 lower one being very variable in size, so that we have found 

 it impossible to draw any clear distinction between those forms 

 with three lateral crenations and those with four. The lateral 

 and apical crenations. as in so many Desmids, are merely 

 the optical expression of ridges which run across the cell 

 transversely to the greatest breadth. These ridges are them- 

 selves broken up into hummocks, so that one or two series of 

 crenations may often be focussed within the marginal ones. 

 Sometimes each creuatioii is minutely bigranulate, but this is 

 by no means a constant character even in the two halves of 

 the same cell. The vertical ribs in the centre of the semicell 

 are also very variable, sometimes beino- scarcely evident and 



i/ O / 



at other times more or less distinctly granulated. 



After a careful consideration of the variation exhibited by 

 C. crenatuni we have been compelled to include within the 

 general description of the species Boldt's forma tricrenata 

 and Schmiclle's var. psychophilum (founded upon Xordstedt's 

 Spitzbergen forms) and var. subls&vis. 



It should also be mentioned that C. crcnatum varies much 

 both in size and in general proportions. 



Forma Boldtiana (Gutw.) nolt. (PI. XCVIII, figs. 



13, 14.) 



Cosmarium Boldtianum G-utw. Flor. Grlon. Okolic Tarnapola, 1894, p. 100 



t. 3, f. 36 ; Sehmidle, Alg-. axis Sumatra, 1895, p. 30-4. 

 C. tricrenatum Gutw. Prodi*. Flor. Alg. Gralic. 1895, p. 351. 



Semicells with 5 or (j crenations on each lateral 

 margin, the two lowest ones often much reduced in 

 size. 



Length 29-42 p. ; breadth 22-28 p. ; breadth of 

 isthmus 10-14 p,; thickness 10-18 /x. 



SCOTLAND.- -Not uncommon, but scarce (Hoy $ 

 Bissett). 



. Distribution. --Qa&Gia, in Austria. Sumatra. 



This form only differs from the more typical forms in its 

 lateral crenations. It was originally figured by Ralfs (' Brit. 

 Desm.' 1848, t. 15, f. ln\ 



