158 MOLLUSCA. GASTEROPODA. 



separated, ornamented with transverse ribs or sometimes 

 with longitudinal lamellae. Aperture circular, entire, 

 margin thickened. Operculum horny, paucispiral. Trias 

 to present day. 



Turritella. Shell imperforate, turreted, many- 

 whorled, whorls flat or slightly convex, ornamented with 

 spiral ribs and with striae of growth; spire very long. 

 Aperture oval or subquadrate, entire, outer lip thin. 

 Operculum horny. Trias to present day. 



Calyptraea. Shell conical, limpet-shaped, spiral, apex 

 central ; interior with a half-cup shaped process under the 

 apex and attached posteriorly. Cretaceous to present 

 day. 



Natica. Shell oval, globular, generally smooth, spire 

 short, last whorl very large. Aperture semilunar or oval, 

 entire ; outer lip sharp, inner lip callous, not crenulate. 

 Frequently an umbilicus, often filled with callus. Oper- 

 culum of same size as aperture, horny or calcareous, 

 paucispiral, nucleus excentric. Carboniferous (perhaps 

 Silurian) to present day. 



Paludina. Shell thin, turbinate; whorls convex, 

 smooth or with faint ribs. Aperture entire, oval, slightly 

 angulated behind. Operculum horny with concentric 

 striae. Lower Cretaceous to present day. Lives in fresh- 

 water. 



Littorina. Shell thick, not nacreous internally, tur- 

 binate, few-whorled, imperforate. Aperture rounded, an- 

 gular behind, outer lip sharp. Columella flattened. 

 Operculum horny, paucispiral. Lias to present day. 



Pseudomelania. Shell many-whorled, elongate, tur- 

 riculate, imperforate, spire long. Aperture oval, entire, a 



