PTEROPODA. 503 



Class PTEROPODA.* 



Head more or loss distinct ; eyes none ; month often fnrnished 

 with cup-shaped appendages. Fins two on the sides of the mouth ; 

 or two, or rarely four, on the side of the body between the head and 

 abdomen, often furnished with a small intermediate lobe between 

 them, ai)parently the rudiment of the foot of Gaslcropuds. Body 

 ovate or roundish, often enclosed in a thin, conical, cylindrical, or 

 sul)-globular shell, with a transverse contracted mouth. Individual 

 uni-sexual ? Animal free, floating on the surface of the sea by the 

 assistance of its fins. Nocturnal or crepuscular. 



Order THECOSOMATA. 

 Head indistinct, with two wings on the sides of the mouth. 

 Tooth of lingual membrane hooked, with a strong hooked tooth 

 on each side. Gills internal. Body enclosed in a shell. 



Family CAVOLINID.E. 



Animal with two united fins without any posterior foot-like ap- 

 pendage between them. Abdomen voluminous. Gills in pairs. 

 Internal superior organs of generation on the right side. 



Shell calcareous, symmetrical, elongate, or globular. 



Oeniis DIACRIA, Gray. 1840. 



Body short, sometimes with lateral appendages. 



Shell globular ; mouth narrower than the cavity, with a slit on 

 each side, not interrupted in front ; apex often truncated in the 

 adult 



* Dr. Gould had prepared nothintjf on the Pteropoda. I am responsible for all that re- 

 lates to them. The descriptions of families and genera are from II. and A. Adams. 



W. G. B. 



