392 DESMIDE^. 



This species differs so considerably in the form of the 

 marginal cells, that it is difficult to conceive it to be merely 

 a variety of that species. None of Ehrenberg's figures 

 come near to it. PI. xi. fig. 5. d. e. represent distinct species 

 of Pediastrum, which have not been described as such, and 

 the first of which might be named Pediastrum crihriforme, 

 and the second P. lunare. See Plate xcii. figs. 3, 4. Se- 

 veral other species of Pediastra are represented in Ehren- 

 berg's figures, and confounded Avith other species. These I 

 propose to figure in an appendix to this genus, in the hope 

 that they may attract the attention of those who study the 

 beautiful tribe of Desmidece. 



65. SCENEDESMUS Meyen. 



Char. Frond composed of from two to ten cylindrical, fusi- 

 form or oblong cells disposed in one or tivo series, outer 

 ones often lunate. 

 Derivation. From aKrjvrj, a tent, and Ssct/jlo^, a bond. 

 Arthrodesmus Ehrenberg, in part. 



1. SCENEDESMUS QUADRICAUDATUS Breb. 



Plate XCII. Fig. 12. 



Char. Cells usually four, but sometimes there are eight in 

 each frond, rounded at the ends, disposed in a single 

 series, each extremity of the ttvo external cells prolonged 

 into a bristle. 



/Q Cells rather small, external ones with a bristle at each 

 extremity, and one at the outer margin. 



7 ecornis Ehr. — External cells icithout bristles. 



Breb. Alg. Falaise, p. 66. Menegh. Syn. Desmid. in Liu- 

 nsea, 1840, p. 206. Arthrodesmus quadricaudatus Ehr., 

 Infus. p. 150. t. 10. fig. 16.; Pritch. Infus. p. 189.; 

 Ralfs, in Annals, vol. xv. pi. xii. fig. 4. ; Jenner, in Fl. 

 of Tunbridge Wells, p. 200. — 7 ecornis. Scenedesmus 



