and 1 antapical plate; chromatophores golden-brown fusiform bod- 

 ies, radially arranged. 



The plates in this genus can be discerned only under favorable 

 optical conditions. 



Hemidiniiim nasutum Stein 1883, p. 91 

 PI. 90, Figs. 4-6 

 Cells elliptical or narrowly ovoid; transverse furrow incomplete, 

 curving dov^m to the right as seen from the ventral side; longitudinal 

 furrow forming a narrow sulcus which extends from the transverse 

 furrow to the posterior pole which is broadly rounded; cell 16-20|a 

 in diameter, 24-28/^ long. 



Fairly common in the plankton of several lakes, especially in 

 marginal waters among dense beds of other algae. Wis. 



FAMILY PERIDINIACEAE 



This family has cells which are globose, or only slightly flattened, 

 fusiform, or top-shaped. The transverse furrow is broad and hori- 

 zontal, definitely dividing the cell into epicone and hypocone. 

 Although usually broadly rounded anteriorly, the epicone may be 

 narrowed and pointed at the apex. The hypocone likewise is usually 

 broadly convex, but in a few forms it may be somewhat produced 

 to form one or more lobes or conical projections. Reproduction is 

 by cell division. Resting cysts of specific shape are used during 

 periods of adverse environmental conditions. 



PERIDINIUM Ehrenberg 1832a, p. 38 

 [Peridinium Ehrenb., Stein 1883] 

 Cells globose, ovoid, or fusiform ( in fresh- water species ) , usually 

 somewhat dorsiventrally flattened, either broadly rounded at the 

 poles or produced to form apiculations or short horns; transverse 

 furrow infra-median and slightly spiral, the epicone sometimes with 

 a true apex and a pore at the anterior pole, or with a false apex 

 that is produced into a horn without a pore and is longer than the 

 hypocone; longitudinal furrow usually broad in the hypocone and 

 extending to the posterior pole, or not, and into the epicone slightly; 

 flagella attached in the ventral sulcus, one winding about the cell 

 in the transverse furrow, the other trailing; arrangement and num- 

 ber of plates in the epitheca variable, usually 4 apical, 3 intercalary 

 and 6-7 precingular plates, one of the apical plates extending from 

 the top of the longitudinal sulcus to the apical pole (the rhomboid 

 or ventral plate); hypotheca with 5 postcingulars and 2 antapical 



[431] 



