284 DEscKiPTiON OF {Tolygasirk'a. 



CtosTEnrcTM; arciiatum. — Smooth, slender, crescent-shaped, ends 

 obtiise, scarcely notched. 



C. Venus (Kiitzing.) — Mr. Ealfs is unable to discover any characters 

 sufficient to distinguish it from C. Diame. 



C. amhhjonema (Ehr.) — Stout, linear, slightly curved, twenty times 

 longer than broad ; ends roiinded. 



C. uncinaticm. — Slender, finely and closely striated ; extremities 

 tapering to a sub-acute point, and suddenly curved downwards. 



C. decussatum. — Stout, finely and closely striated, slightly curved, 

 gradually tapering; extremities slender, but obtuse at the apex. 

 It seems to diiFer from C. turgichim in its more tapering extremities. 



C. ttirgidulum. — Stout, curved; extremities slender, gradually 

 tapering ; striee few, conspicuous. It differs from C. costatum in its 

 more elongated extremities. 



C. oMiisangulum. — Crescent-shaped; four- sided ; angles rounded; 

 ends rounded. Empty frond transparent, colourless, with a narrow 

 central suture, 



C. incequale (Ehr.) — Miaute, semi-lunate ; extremities unequal, 

 conical, ends acute ; striae prominent ; vesicles scattered. 



C. quadrangulare. — Long, thin, quadrangular, the angles acute ; 

 endochrome deep gi-een ; sutures several, oblique ; ends rounded. 



C. gracile. — Slender, smooth, lanceolate, gradually tapering into 

 short beaks, which are curved downwards. It diiFers from other 

 rostrate species in its smooth frond. Its beaks are shorter than 

 those of C. rostratum, and C. setacetim, and it is less inflated. 



C. tenerrimuni (Kiitz.) — Mr. Ralfs states he can perceive, in Prof. 

 Kutzing's drawing, no character by which it can be separated from 

 C. acufum- 



Genus Spirot.^nia (Brebisson.) — Frond simple, elongated, cylin- 

 drical, or fusiform, straight, entire, not constricted at the middle ; 

 ends rounded; endochrome spiral, as in i^^«(?»m; this circumstance 

 distinguishes it from every other genus. It differs from Tefme- 

 morus in the entire extremities, and absence of central constriction ; 

 and, in the latter, likewise from Chsterium. The extremities are 

 also without globules. It closely resembles Zygnema in its endo- 

 chrome, but docs not form a filament, the ccUs dividing in the 

 manner of the Des^nidiece, but obliquely as in Scenedesmus. In Spi- 

 rotcenia complete division of the cell is prior to the division of the 



