156 MOLLUSCA. GASTEROPODA. 



Patella. Shell conical, oval or sub-circular; apex 

 sub-central or excentral, recurved anteriorly ; surface with 

 radiating ribs or striae, rarely smooth. Margin simple or 

 spinose. Muscular impression horse-shoe shaped, open in 

 front. Jurassic to present day. 



Emarginula. Shell conical, surface cancellated, 

 apex recurved posteriorly. Anterior border with a well- 

 marked slit. Muscular impression horse-shoe shaped. 

 Carboniferous Limestone to present day. 



Pleurotomaria. Shell trochiform, conical, turbinate, 

 or discoidal ; interior nacreous. Umbilicus present or 

 absent. Aperture subquadrate or oval, outer lip sharp, 

 with a slit, which as the shell grows becomes filled up, 

 leaving a band on the whorls, toward which the lines of 

 growth converge, being directed obliquely backwards. 

 Operculum horny. Upper Cambrian to present day. 



Murchisonia. Shell turreted, with many whorls, 

 provided with a band as in Pleurotomaria, Aperture 

 oval, with a slit, and a short anterior canal. Ordovician 

 to Permian. 



Bellerophon. Shell globular or discoidal; whorls 

 few, symmetrically coiled in one plane. Aperture sub- 

 circular or oval, with a deep median sinus, which is 

 replaced by a band or keel dividing the shell into two 

 similar parts; columellar edge often with callus. Upper 

 Cambrian to Permian. 



Fhasianella. Shell elongated, oval or oblong, 

 smooth, polished, imperforate, interior not nacreous. 

 Aperture oval, entire, rounded anteriorly, angular pos- 

 teriorly ; outer lip thin, simple, sharp. Columella 



