244 MOLLUSCA. GASTEROPODA 



bilicus, turreted, without periostracum. Whorls numerous, narrow, 

 the last whorl always much shorter than the spire. Aperture 

 oblong or semi-oval, with a short posterior canal and a well-marked 

 recurved anterior canal ; outer lip more or less thickened and often 

 somewhat reflected ; columellar edge concave. Operculum horny, 

 oval, paucispiral, with submarginal nucleus. Jurassic to present 

 day. Ex. C. adansoni, Recent ; C. mutabile, Barton and Brackle- 

 sham Beds. There are several sub-genera. Cerithium in the 

 restricted sense includes forms in which there is a strong ridge 

 on the posterior part of the inner lip forming the inner boundary 

 of the posterior canal, the outer lip is expanded in front, and the 

 whorls are provided with varices. Ex. C. nodulosum, Recent. 



Potamides. Form similar to Cerithium. Shell with a brown 

 or blackish periostracum. Aperture* rounded or sub-quadrangular ; 

 either with a fold in front or a very short and not recurved 

 anterior canal ; outer lip rather thin. Operculum circular, multi- 

 spiral, with central nucleus. Lives in brackish water. Cretaceous 

 to present day. Ex. P. funatus, Woolwich Beds ; P. lamarchi, 

 Oligocene. 



Aporrhais. Shell fusiform, without umbilicus, whorls nume- 

 rous, spire elongate. Aperture produced into a straight or curved 

 anterior canal. Outer lip expanded, thick with an anterior sinuosity, 

 lobed or digitate, one of the processes being attached to a part of 

 the spire. Operculum small. Jurassic to present day. Ex. A. pes- 

 pelicani, Coralline Crag to present day. 



Dicroloma ( = Alaria). Similar to Aporrhais, but without a 

 process from the outer lip attached to the spire, and without 

 anterior sinuosity. Jurassic and Cretaceous. Ex. B. armata, 

 Great Oolite. 



Strombus. Shell ovoid, ventricose, tuberculate or spiny, 

 without umbilicus. Spire with several whorls. Last whorl very 

 large. Aperture long, narrow, with a short anterior channel ; 

 canaliculate posteriorly ; outer lip expanded, wing-like, thick, often 

 lobed behind, with a sinus near the anterior margin. Operculum 

 small, horny, claw-shaped, with serrated edge. Eocene to present 

 day. Ex. S. pugilis, Recent. 



Rostellaria. Shell fusiform, spire elongated, composed of 

 many whorls, which are smooth or faintly ribbed. Aperture oblong, 



