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 with 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, 



Q6. SCENEDESMUS Meijen. 



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

 form or ohlong cells disposed in one or tico series, outer 

 ones often lunate. 



Derivation. From aKrjvr], a tent, and BecrfjL09, 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 two external cells prolonged 



into a bristle. 



P Cells rather small, external ones loith a bristle at each 



extremity, and one at the outer margin. 

 7 ecornis Ehr. — External cells without bristles. 



Breb. Alg. Falaise, p. ^Q>. Mencgh. Syn. Desmid. in Lin- 

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

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

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

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



