No. IO.] FRESH-WATER ALG^. 63 



E. sp. (?), Fig. 227. 



E. ansatum (Ehrb.) Ralfs, found but not figured. 



Micrasterias Ag. (Holocystis Hass. ; Tetrachastrum 

 Dixon) . Cells simple, flattened, in the form of a double-con- 

 vex lens, deeply constricted in the middle. Front view 

 orbicular or broadly elliptical ; end view spindle-shaped, with 

 acute ends. Each semi-cell three- to five-lobed ; lateral lobes 

 entire, or irregularly cut into large, deep lobes ; the end lobes 

 entire, or more slightly cut, sometimes with angles pro- 

 nounced, and two-cleft. Zygospores seldom found, large, 

 globular, with stout spines which are at first simple and later 

 branched. 



M. radiosa (Ag.) Ralfs var. punctata West, Fig. 106. 



M. apiculata Menegh., Fig. 108. 



M. rotata (Grev.) Ralfs, Fig. 238. 



M. furcata (Ag.) Ralfs, Fig. 239. 



M. Americana (Ehrb.) Kiitz., Fig. 240. 



M. muricata Bailey, Fig. 241. 



M. truncata (Corda) Ralfs, Fig. 242. 



M. Crux-Melitensis (Ehrb.) Hass., Fig. 107. 



Arthrodesmus Ehrb. Cells simple, deeply constricted in 

 the middle; each half-cell is wider than long, and furnished 

 with several spines. This genus is distinct from Xanthidium 

 by the absence of the protruding area in the centre of the semi- 

 cells. There are usually fewer spines, and the zygospores are 

 either smooth or spinous. 



A. octocornis Ehrb., Fig. 260. 



A. convergens (Ehrb.) Ralfs, Fig. 259. 



Cosmarium Corda ( Dysphinctium Nag.; Calocylin- 

 drus (Nag.) Kirch.; Cosmaridium Gay; Pleurotaeniopsis 

 (Lund.) Lagerh.). The largest genus of Desmids. The 

 single cells circular, elliptical, or oblong, usually one and one- 

 half diameters in length, always more or less constricted in 

 the middle. Ends usually entire, rounded or truncate. The 

 margins are smooth, dentate, or crenate, the cell wall smooth, 

 punctate, warty, or even covered with spines. End view 

 oblong or oval, with sometimes a swelling in the middle of 

 the longer sides. Zygospores usually spherical, sometimes 

 cubical or angular, smooth or furnished with spines. 



