554 Annals of the Smith African Museum. 



d, g). As the figures show, there are often appreciable differences 

 between the two semicells, and even between the two sides of a semi- 

 cell, in both these respects. 



Other variations concern the upper lateral margins. They may lack 

 the median crest and have a more or less concave edge (Fig. 28,/, g), 

 which may sometimes, however, be almost straight (Fig. 28, g, the 

 right-hand side of the lower semicell). In this respect also there may 

 be diversity on the two sides of the same semicell. 



Lastly, the apex varies considerably in breadth and in the degree of 

 development of the refuse character, being sometimes practically 

 truncate (Fig. 28, d-g). 



In those forms, in which the lower lateral margins become sub- 



Fxg. 2S.— a, h. Cosmarium sexangulare, Lund., var. subangulare, nov. var. 

 c,C. rectangulare, Grun., forma. </-/<. Cosmarium pervusillum, West, 

 formae (h, end-view). All figures x 800. 



parallel, the cell is proportionally rather longer than broad (long., 

 20-21 fjr, lat., 16-17 /a). Such specimens (Fig. 28, e, /), except for the 

 frequent presence of the median crest on the upper lateral margins, 

 approach very closely to 0. meneghinii, Brcb., and it may be questioned 

 whether C. perpusilhim, West, would not better be placed as a variety 

 of the former species, since it appears to be connected with it by a 

 whole series of transitional forms in the South African flora, Forms 

 somewhat resembling those here descri bed have been recorded under 

 C. meneghinii, Breb., by Borge (Alg. d. ersten Regnell. Exped., ii. 

 Desmid., Arkiv. f . Bot. i, 1003. p. 08. Tab. Ill, fig. 24), and Raci- 

 borski (De nonnull. Desmid. Polonia, Pamietnik Wydz. Ak. Umiej. w. 

 Krakow, x, 1885, Tab. XI. fig. 5) ; cf. also var. nanum, Wide (W. A 

 CI. S. West, op. cit., p. 93, PI. LXXII, tig. 34). 



It should be added that there are no appreciable differences in 

 the side- and end-views of the specimens shown in Fig. 28, d-g, 



