epicone about twice the length of the hypocone sq that the trans- 

 verse furrow is inframedian; longitudinal furrow narrow, extending 

 about half way into the epicone, broader in the hypocone; 27-30ju, 

 in diameter, 40-60/t long. 



Mich. 



ORDER PERIDINIALES 



In this group the cells have a definite, thick membrane which is 

 constructed of plates (see p. 424). There is a definite epitlieca and 

 hypotheca separated by a broad transverse furrow which sometimes 

 completely encircles the cell. In most of the fresh-water species the 

 plates are thick and are separated by wide sutures, but in one 

 family (Glenodiniaceae) the plates are thin and close-fitting. 



Key to the Families 

 1. Cell wall thin, plates obscure glenodiniaceae 



1. Cell wall with conspicuous plates, usually separated by sutures 2 



2. Cells with anterior half extended into a long, 



horn-like process.— ceratiaceae 



2. Cells with anterior half broadly rounded or 



conical but not extended into a horn peridiniaceae 



FAMILY GLENODINIACEAE 



These organisms are globose or somewhat flattened dorsi\'entrally, 

 ellipsoid or top-shaped in front view. The cells have a very thin 

 wall in which there are delicate, scarcely discernible plates. There 

 are numerous brown chromatophores and a pigment-spot is usually 

 conspicuous. The transverse furrow does not extend completely 

 around the cell in some forms. 



Key to the Genera 



With a transverse furrow that extends completely 



around the cell Glenodinium 



With a transverse furrow that extends only 



part way around the cell ' Hemidinium 



GLENODINIUM (Ehrenb.) Stein 1883, p. 91 



Cells globose (very slightly flattened dorsiventrally ) , with a very 

 thin theca in which there are faintly marked-out plates separated 

 by narrow sutures (best seen in plasmolyzed or stained specimens); 

 transverse furrow complete, either in one plane or shghtly spiral in 

 some species; epicone broadly rounded or somewhat apiculate, with 

 a variable number of plates; hypotheca with 5-6 postcingulars and 

 usually 2 antapical plates (rarely 1); chromatophores numerous, 



[427] 



