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BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



of a few numerically rare and structurally widely divergent species, fall 

 into five sharply denned subgenera as follows : — 



Tripoceratium, subgen. nov. 



Antapical horns subequal, reflected anteriorly, their tips symmetrically pointed 

 closed. Apical horn differentiated abruptly from the rotund midbody. Post- 

 margin rounded, no postindentation. Over 25 species. C. tripos, C. arcuatum 

 (fig- A). 



Macroceratium, subgen. nov. 



Antapical horns, subequal, reflected anteriorly, their tips truncate, open, or con- 

 tracted or rounded, but usually with terminal pore. Apical horn differentiated 

 abruptly from more or less rotuud midbody. Bases of the antapicals projected 

 more or less posteriorly beyond midbody forming a postindentation. Over 25 

 species. C. macroceros, C. gallicum (fig. B), C vultur (plate 4, fig. 7). 



Biceratium Vanhoffen. 



Antapical horns more or less unequal, deflected posteriorly, their tips pointed, 

 closed. Apical horn differentiated or tapering from the midbody. Deep post- 

 indentation. Over 10 species. C-furca, C. pentagonum (fig. C), C. californiense 

 (plates 1-3, plate 4, fig. 4). 



Fig. A. Ceratium arcuatum, dorsal view. X 155. 

 Fig. B. C. gallicum, ventral view. X 155. 

 Fig. C. C. pentagonum, dorsal view. X 155. 



