MOLLUSCA. GASTEROPODA. 159 



little inflated in front, narrowed behind ; outer lip sharp. 

 Columella smooth. Lias to Tertiary. 



Melania. Shell elongate, turreted, many-whorled, 

 imperforate, apex sharp but usually corroded. Surface 

 smooth, ribbed or spiny. Aperture entire, narrow behind, 

 rounded in front; outer lip sharp, slightly sinuous. 

 Operculum horny, subspiral. Wealden to present day. 

 Lives in freshwater. 



Nerinea. Shell elongate, perforate or not, whorls 

 numerous. Aperture sub-quadrangular, oval or elongate, 

 with a short anterior canal and a posterior slit near the 

 suture, which forms as it becomes filled up a continuous 

 band. Columella and also the interior of the whorls 

 furnished with folds which are continuous to the apex. 

 Inferior Oolite to Upper Cretaceous. 



Cerithium. Shell imperforate, turreted, without epi- 

 dermis. Whorls numerous, the last whorl always shorter 

 than the spire. Aperture oblong or semi-oval, with a 

 short posterior canal and a well-marked recurved anterior 

 canal; outer lip expanded. Operculum horny, oval, 

 paucispiral. Trias to present day. 



Strombus. Shell ovoid, ventricose, tuberculate or 

 spiny, imperforate. Spire with numerous whorls, the last 

 very large. Aperture long, narrow, with a short anterior 

 canal ; often canaliculate posteriorly ; outer lip expanded, 

 thick, often lobed, with a deep notch near the anterior 

 canal. Operculum small, horny, claw-shaped, edge serrated. 

 Lower Cretaceous to present day. 



Rostellaria. Shell fusiform, spire elongated, com- 

 posed of many whorls, which are smooth or faintly ribbed. 

 Aperture oblong with a long straight or slightly curved 



