plates; all plates conspicuously marked with reticular thickenings 

 and sometimes other decorations, such as small spines, and with 

 narrow or wide sutures between the plates which are usually 

 striated, as is also the transverse furrow; in some species with a 

 conspicuous flange or wing-like rim about the cell through which 

 small ribs or concretions radiate to the margin from the surface of 

 the plates. 



Key to the Species 



1. Cells broadly fusiform, extended posteriorly into 1 or more processes 2 



1. Cells broadly rounded posteriorly 3 



( See P. cinctum var. tuberosum, however. ) 



2. One posterior horn-like extension P. wisconsinense 



2. Two posterior horn-like extensions P. limbatum 



3. Cells 13-20m in diameter 6 



3. Cells larger 4 



4. Cells nearly circular in outline - 5 



4. Cells distinctly longer than broad.. P. Willei 



5. Cells 35— 55/x in diameter; longitudinal furrow extending 



for some distance into the epicone P. cinctum 



5. Cells 55-80/i in diameter; longitudinal furrow 



scarcely extending into the epicone P. gatunense 



6. Cells with 3 small teeth at the posterior pole P. inconspicuum 



6. Cells with broadly rounded smooth antapical poles P. pusillum 



Peridinium cinctum ( Muell. ) Ehrenberg 1838, p. 253 



PI. 91, Fig. 1-4 



Cells globose, subglobose, or broadly ovoid in ventral view, very 

 slightly flattened dorsiventrally as seen in polar view; transverse 

 furrow broad, spiral, dividing the cell almost equally on the left 

 side (as seen ventrally) but spiralling to a supramedian position 

 on the right; plates thick and coarsely reticulate; epicone high and 

 broadly rounded, epitheca with 4 apicals (including the rhomboid 

 plate), 3 intercalary, and 7 precingular plates; hypocone broadly 

 rounded posteriorly, hypotheca with 5 postcingular and 2 antapical 

 plates; cells 35-55jU. in diameter, 40-60 fi long. 



Fairly common in both euplankton and tychoplankton. Mich., Wis. 



Peridinium cinctum var. Lemmermannii 

 G. S. West 1909, p. 190 



Cells 62-70/A in diameter, 56-70/i, long, 52-53/* thick; a variety in 

 which the right antapical plate is distinctly larger than the left. 

 Euplanktonic and tychoplanktonic. Mich., Wis. 



[432] 



