PTEROPODA. 503 



Class PTEROPODA. 



* 



Head more or less distinct ; eyes none ; mouth often furnislied 

 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, apparently the rudiment of the foot of Gasteropods. Body 

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

 sub-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. 

 Tootli of lingual membrane hoolted, with a strong hooked tooth 

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



Family CAVOLINIDiE. 



Animal wnth 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. 



Oenus 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 nothinpj on the Pteropoda. I am responsible for all that re- 

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



W. G. B. 



