MOLLUSCA—PTEROPODA. 



15 



Class PTEROPODA Cuvier. 

 Free-swimming mollusks, with or without a thin, transparent 

 shell. No distinct head present. Eyes rudimentary and foot 

 replaced by two lateral, wing-like fins on the anterior end of the 

 body. Body sometimes straight, sometimes coiled posteriorly into 

 a spiral. Many shell-covered forms develop a horny operculum. 

 Shell mostly transparent and very variable in form (Fig. 1228). 



Fig. 1227. «, by Conularia newberryi, X |^> with enlargement of surface ; c-f^ C. 

 micronema, X H^ '^'\^ enlargement of surface. (Pal. Ohio.) 



Pteropods lead a pelagic life, rising in vast numbers to the sur- 

 face of the sea toward nightfall. The shells accumulate on the 

 ocean floor, where they form pteropod oozes. 



The class is most typically represented in Mesozoic and later 

 deposits. In the Palaeozoic they are represented by the Devonic 

 StyliolinidcE. 



For literature see under Conularida. 



I. Styliolina Karpinsky. 



Shells small, needle-shaped, with a circular cross section. Apex 

 solid and usually bulb-shaped. Surface smooth, marked only with 

 fine lines of growth. Devonic. 



