MOLLUSCA. 



FAM. 9. CERITHIAD^E. Shell spiral, turreted ; aperture 

 channelled in front, with a less distinct posterior canal. Lip 

 generally expanded in the adult. Operculum horny and 

 spiral. The Cerithiadcz are exclusively confined to the Second- 

 ary, Tertiary, and Recent periods, and are represented in the 

 Tertiary Rocks by a vast number of forms. The most import- 

 ant fossil forms belong to the genera Cerithium, Pvtamides, 

 Nerincea, and Aporrhais, of which Nerinaa is extinct, and is 

 exclusively confined to the Secondary period. 



For all practical purposes Cerithium and Potamides may be 



considered together, as no 

 strict line of demarcation 

 can be drawn between the 

 fossil forms. In both, the 

 shell is turreted and many- 

 whorled (fig. 215), with or 

 without varices. 



The aperture of the shell 

 is small, with a tortuous an- 

 terior canal, and an ex- 

 panded outer lip. Most of 

 the living forms are inhabi- 

 tants of fresh or brackish 

 waters, and they are chiefly 

 found in hot climates. The 

 fossil forms, to the number 

 of nearly five hundred, 

 commence in the Trias, but they attain their maximum of 

 development in the Eocene Tertiary. 



In the genus Nerincea (fig. 2 1 6), the shell is turreted, many- 

 whorled, and nearly cylindrical. The columella carries con- 

 tinuous ridges, and similar ridges exist on the interior of the 

 whorls, so that casts of the interior of the shell are often very 

 unlike the form of the exterior. The aperture of the shell is 

 channelled in front. The species of Nerincea are exclusively 

 Jurassic and Cretaceous, and are very numerous. One of the 

 limestones of the Jura, believed to be of the age of the Coral 

 Rag (Middle Oolite) of Britain, abounds to such an extent 

 in these shells as to have gained the name of " Calcaire a 

 Nerinees." 



In the genus Aporrhais, lastly, the shell is turreted, and the 

 outer lip of the adult is greatly expanded and lobed. The 

 species of this genus are marine in their habits. A great many 

 Jurassic and Cretaceous shells, generally referred at present to 

 Rostdlaria, probably belong really to Aporrhais ; but the de- 



Fig. 215. Cerithi- Fig. 216. Nerin&a 

 inn liexagonum. Eo- bisulcata. Chalk, 

 cene Tertiary. 



