MOLLUSCA. GASTEROPODA. 157 



flattened. Operculum calcareous, convex externally, 

 concave internally. Devonian to present day. 



Turbo. Shell solid, turbinate or conical, whorls 

 convex, interior nacreous. Aperture large, circular, entire, 

 slightly produced anteriorly; outer lip sharp. Imper- 

 tbrate or with a small umbilicus. Operculum thick, 

 calcareous, exterior convex, interior flat and spiral, 

 nucleus central or sub-central. Silurian to present day. 



Trochus. Shell conical, whorls numerous and flat or 

 slightly convex, the last whorl angular; base flat or nearly 

 so. Aperture oblique, rhomboidal, outer lip sharp. 

 Columella twisted, with a prominent anterior protuber- 

 ance. Operculum horny, multispiral. Silurian to present 

 day. 



Nerita. Shell thick, solid, ovoid or semiglobose, 

 imperforate. Spire short. Surface smooth or with spiral 

 ribs. Aperture semicircular; outer lip thick, denticulate; 

 inner lip callous, flattened with a straight denticulate 

 border. Operculum calcareous. Cretaceous to present 

 day. 



Solarium. Shell conical, depressed, angular at the 

 periphery. Aperture entire, subquadrate ; lip sharp. 

 Umbilicus wide and deep, limited by a crenulated margin. 

 Operculum horny, spiral. Cretaceous to present day. 



Euomphalus. Shell depressed, discoidal, with a 

 wide and large umbilicus ; whorls convex with a ridge on 

 the upper surface. Aperture polygonal ; outer lip with a 

 sinus on its upper surface. Silurian to Carboniferous. 



Scalaria. Shell more or less umbilicate, turreted, 

 spire elongate ; whorls numerous, convex, sometimes 



