92 BRITISH DESMIDIEiE. 



authorities, and pointed out the characters which distinguish those 

 genera from Cosmarium. 



Both Ehrenberg and Meneghini consider the inflato-compressed 

 segments essential; but some species with globular or cylindrical 

 segments can by no means be separated from the compressed ones. 

 Those having cylindrical fronds in some respects show an affinity with 

 Penium ; but in that genus the constriction is either wanting or ob- 

 scure, and the segments are longer than broad. In Cosmarium the 

 fronds are never elongated, are always constricted in the middle, and 

 the starch vesicles are scattered. 



Tetmemorus differs from this genus in its elongated fronds and 

 emarginate extremities. 



The species belonging to Cosmarium may conveniently be divided 

 into three sections. 



In the first section the segments are compressed, and in the front 

 view, which differs from the lateral one, they are united by a portion 

 only of their bases, and thus the constriction forms a linear notch on 

 each side. An end view is elliptic. 



The plants belonging to the second section are also compressed, 

 and, like those in the preceding, their front view differs from the 

 lateral one and has a linear notch on each side. Each segment how- 

 ever has a central inflation, which causes the end view to be more or 

 less cruciform. 



In the third section the end view is circular, the front and lateral 

 views are alike, and there is no linear notch at the sides, since the 

 segments have no constriction at their junction except that which 

 necessarily results from their figure. 



* Frond compressed, deeply constricted at the middle ; end view 



elliptical. 

 t Margin of segments entire. 



1. C. quadratum (Ralfs) ; frond smooth, deeply constricted at the 

 middle ; segments in the front view quadrate, and on each side of 

 the base slightly protuberant. 



Cosmarium quadratum, Ralfs, Annals of Nat. Hist. v. 14. p. 395. t. 11. f. 9. 

 (1844) ; Trans, of Bot. Soc. of Edinburgh, v. 2. p. 151. t. 16. Hass. 

 Brit. Alg. p. 3G7. 



Dolgelley, J. R. Rusthall Common near Tunbridge Wells, Mr. Jenner. 

 Aberdeenshire, Br. Dickie and Mr. P. Grant. 



Falaisc, Brchisson. 



