The Freshwater Algae of Africa. 553 



Forum apice leviter retuso, membrana levissime punctata. Long., 

 fe9/x.; lat.. 25 ,,„ 



Sample 7 (rare). 



Compare with forms described by Lundell fDesra. Suec, loc. cit., 

 p. 47) <md Nordstedt (Freshw. Alg. New Zealand, etc., K. Sv. Vet.-Ak. 

 Handl., xxii. 1888, p. 55, Tab VI. fig. 5). 



7. Cosmarium sexangulare, Lundell, Desm. Suec., loc. cit., p. 35, 

 Tab. II, fig. -2W ; W. & (i. S. West. op. cit. iii, 1908, p. 81, PI. LXXII. 

 fig. 3. 



Var. subangulare, nov. var. (Fig. 28, ". b). 



Magnitudine ca. dimidio ea typi ; semieellulis obscure 7-angulis, 

 lateribus superioribus e marginibus duabus fere rectis et obscure 

 delimitatis constantibus ; angulis lateralibus bene demarcatis, rotun- 

 datis vel subacutis ; apicibus interdum leviter retusis. 



Long, cell., 18-25 // •. lat.. 15-21 // ; lat. isthm., 4-5/*; ci'ass., 12 fi. 



Samples 29, 30a, 31, 39. 



This variety is distinguished by its refuse apex, and especially by 

 the fact that the upper lateral margins consist of two, almost flattened 

 edges, forming a Tery wide angle with one another (Fig. 28, 6). In 



the former respect it resembles forma minima, Nordst., with which it 

 also agrees to some extent in its dimensions; in this form, however, 

 the upper lateral margins are refuse. 



8. Cosmarium perpusillum, West, Alg. W. Ireland, Journ. Linn. Soc, 

 Bot., xxix. 1892, p. 148, Tab. XXI. fig. 2: W.& G. S. West. op. cit., 

 iii, 1908, p. 88. PI. LXXII, figs. 22, 23. (Fig. 28, d h.) 



Samples 29. 29a (very rare). 30a, 31, 39. 



Long., 18—20 ix ; lat., 15-18 ft ; lat. isthm. ,^3-4 fx ; crass.. 9 ft.- 



It appears that the specimens in these samples belong to- the above 

 species, although they exhibit considerable variation in several respects, 

 apart from being larger. Typical specimens have subhexagonal semi- 

 cells, with the lower lateral margins slightly refuse and the upper 

 lateral margins provided with one median crest ; the apex is generally 

 slightly refuse. I have in no case observed so marked a protrusion 

 of the lateral angles, as is figured by Messrs. Wesl in their mono- 

 graph. 



Considerable variation is firsth exhibited in the degree of divergence 

 of the lower lateral margins, which in some cases (Fig. 28, e, f) 

 become almost subparallel, giving the lower pari of the semicell the 

 rectangular appearance characteristic of G. meneghinii, Breb. Further. 

 whilst in many individuals these lower margins are typically retuse 

 (Fig. 28, e), in others they are almost, if not quite, straight (Fie-. 28, 



