MOLLUSC A. GASTEROPODA. 163 



ORDER. PTEROPODA. 



The Pteropods are all pelagic animals. The head is 

 not well marked, and the foot is represented by two 

 lateral wing-like fins. A shell may or may not be present. 



Hyolithes (= Theca). Shell straight, rarely curved, 

 pyramidal, triangular in section, smooth or striated ; 

 posterior part sometimes crossed by walls. Aperture 

 sometimes oblique ; provided with an operculum having 

 the form of a half cone. Upper Cambrian to Permian. 



Conularia. Shell generally straight, pyramidal, sec- 

 tion quadrangular, each lateral face with a median longi- 

 tudinal groove ; posterior part of shell sometimes provided 

 with concave partitions. Surface ornamented with nume- 

 rous transverse, parallel, angulated ridges. Aperture 

 partly closed by triangular or lobed processes. Ordovician 

 to Trias. 



Tentaculites. Shell thick, solid, in the form of a 

 greatly elongated cone, straight or slightly curved, section 

 circular, posterior extremity sharp or with a vesicular 

 enlargement. Surface provided with prominent, trans- 

 verse, parallel rings, and with transverse and longitudinal 

 striae. Ordovician to Devonian. 



Distribution of the Gasteropoda. 



Some of the Gasteropoda live on land, others in fresh 

 water, but the majority are marine ; they are found in the 

 seas of all parts of the world, but are especially abundant 

 in warm regions and in comparatively shallow water. A 

 few forms can exist both on land and in water, e.g. 

 Ampullaria, which commonly lives in lakes and rivers, is 



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