chromatophores which are radially arranged; membrane smooth ( in 

 our specimens) or with longitudinal striations. 



Most of the species of Gymnodinium are marine, but a few of the 

 known fresh-water forms have been recorded for this country. 



Key to the Species 



1. Cells broadly rounded posteriorly, emarginate 



at the antapical pole G. palustre 



1. Cells extended posteriorly to form a cone, or a caudus 2 



2. Cells extended posteriorly into a ctirv'ed caudus G. caudatum 



2. Cells extended into a short, cone-shaped portion posteriorly..- G. fuscum 



Gymnodinium caudatum Prescott 1944, p. 371 

 PI. 90, Figs. 1-^ 



Cells large, ovoid to in^'e^sely conical or top-shaped, broadly 

 rounded at the anterior end, narrowed and produced into a curved 

 caudus at the posterior pole; \eiy much flattened dorsiventrally; 

 transverse furrow prominent and median, spirally turned to the 

 left; chromatophores numerous golden-brown, ovoid, or elongate 

 plates, radially disposed; pigment-spot present near the sulcus in 

 the hypocone;' longitudinal furrow extending about half the length 

 of the hvpocone and for a short distance into the epicone; cell 

 65-70^ in diameter, 104-1 IS^t long. 



The large size of the cell and the narrowed, tail-like hyocone are 

 the distinguishing features of this species. 



In a SpJmgnum bog. Wis. 



Gymnodinium fuscum (Ehrenb.) Stein 1878, p. 95 

 PI. 89, Fig. 23 



Cells large, ovoid, the epicone dome-shaped, the hypocone as 

 broad as or broader than the epicone, narrowed posteriorly to form 

 an inverted cone with a shghtly produced tip; transverse furrow 

 shghtly spiral; the longitudinal furrow extending about half way 

 into the hypocone, but scarcely at all into the epicone; chromato- 

 phores numerous ovoid discs or rods, radially arranged; cells 55-60^ 

 in diameter, 80-100;li long. 



Our specimens are slightly smaller than the dimensions given for 

 G. fuscum but are otherwise in agreement. 



Common in lilv ponds and acid bogs. Wis. 



Gymnodinium palustre Schilling 1891, pp. 248, 277, 278 



PI. 107, Fig. 3 

 Cells relatively small, ellipsoid, the anterior end sharply rounded 

 or somewhat cone-shaped, the posterior pole broadly rounded; 



[426] 



