290 DESCEIPTION OF [Pol^gasirica. 



are inconspicuous. Sccnedesmus differs from the preceding genera in 

 the very different forra of its cells, but Fediastrum supplies a con- 

 necting link between them. As in that genus, the fi-ond in Sccne- 

 desmus is composed of several cells, but these are differently arranged ; 

 and the division into two segments, which, although modified, is 

 still met with in the outer cells of Fediastrum, is entirely absent in 

 Scenedesmus. In the oblique manner in which its cells divide, it 

 agrees with Sjpirotamia, which, however, has a different arrangement 

 of the endochrome, and a frond consisting merely of a single cell. 



Scenedesmus quadricauda ^^ Arthrodestnus quadricaudatus (Ehr.) 

 —Cells, generally four, oblong, rounded at their ends, disposed in a 

 single row ; each extremity of the two external ones usually ter- 

 minated by a bristle (P. 13, f. 19 and 36.) 



Var. fh.j — External cells with three bristles. 



Var. fcj ecornis = Arfhrodesmus ecornis (Ehr.) — All the cells 

 similar and without bristles. Length of cell 1-1 12 1st. Breadth 

 l-2631st. 



S. dimorjpJms := Arthrodesmus pectinatus (Ehr.) — Cells acute, four 

 to eight placed evenly in a single row ; the inner cells fusiform, the 

 outer externally lunate. Frond very minute. Length of cell l-1026th 

 to l-906th. Breadth l-8160th. 



S. acutus = ArtJwodesmus acutus (Ehr.) — Cells two to eight, fusi- 

 form, acuminate, arranged in a single, irregularly alternating scries. 

 The two outer cells are frequently crescent-shaped. Length of cell 

 1-1 063rd to l-1020th. 



S. oUiquus. — Cells eight, elliptic -fusiform ; after division arranged 

 in two distinct, generally oblique series, the outermost cell of each 

 not in contact with any of those in the other series. 



S. ohtusus. — Cells three to eight, ovate or oblong, and arrf Dgel in 

 one row, or, after division, alternately in two rows. Frond minute. 

 Endochrome very pale green. 



This species is rarely met with in a simple state, but, as the cells 

 after division are retained in connection by the mucous matrix, two 

 rows are usually present, the broader ends of one row lying between 

 the cells of the other. The hyaline matrix is fr-equently their only 

 bond of union, and in this state they seem to connect the Desmidiece 

 with the TJlvacccn, through Merismopedia. 



