238 BRITISH DESMIDIACEJ]. 



C. subcostatum is a frequent British species, especially at 

 the weedy margins of lakes and ponds. It is not a Sphagnum- 

 species. It is closely related to C. subcrenatum on the one 

 hand and to C. costatum on the other. In general propor- 

 tions it is very similar to C. subcrenatum, but it differs in its 

 more minute apical crenulations, in its more pronounced 

 bigranulate or emarginate crenations, and in the smaller 

 central protuberance with fewer granules. It is much more 

 easily distinguished from C. costatum, having proportionately 

 shorter cells with a much deeper constriction, more attenuated 

 semicells, less prominent creiiations, and a much smaller 

 central protuberance. 



Forma minor West & G. S. West. (PL LXXXVII, 



figs. 6-9.) 



C. subcostatum Nordst. forma minor West & G. S. West, Alg. Ceiitr. 



Africa, 1896, p. 379, t. 361, f . 15 ; Welw. Afric. Fresliw. Alg-. 1897, p. 



122 ; G. S. West, Alg. Third Tanganyika Expedit. 1907, p. 121. 

 C. Kjellmani var. ornatum Wille forma Borge, Beitrage Alg. Sclrvveden, 



1906, p. 42, t. 3, f. 32. 



Cells rather smaller tbaii in the type, with only 2-3 

 (generally 2) emarginate lateral crenations. Chloro- 

 plasts with one pyrenoid. 



Length 19-24/x,; breadth 18-5-21^; breadth of 

 isthmus 4*2 5*5 /x; thickness 10'5-12*5 /x. 



ENGLAND.- -Near Cockermonth, Cumberland ! 



IRELAND.- -Longh Derryclare, Gal way ! 



Geoc/r. Distribution. Sweden. United States. 

 Central and W. Africa. 



It is very probable that C. subcrenatum Hantzsch var. 

 divaricatum Wille should be referred to this small form of 

 G. subcostatum, as it differs only in its broader central tumour 

 which is furnished with 6-8 vertical series of granules. C. 

 Kjellmani var. ornatum Wille also stands very near, but the 

 sinus is more open and the sides of the semicells are more 

 strongly convergent to narrower apices. 



The C. calcareum figured by Johnson ( c New and Rare 

 Desm. U. S., I/ 1894, t. 211, f. 13) should also be relegated 

 to C. subcostatum forma minor. 



Var. Beckii (Gutw.) nob. (PI. LXXXVII, figs. 10- 



12.) 



Cosmarium Beckii Gutw. Nagj. dosel. Bosni Hercegovin. halugam. 1896, 

 p. 376, t. 1, f. 7 j a. S. West, Alga-fl. Cainbr. 1899, p. 217, t. 395, f. 11. 



