GASTEROPODA. 



17 



Fam. 9. Cerithiad^. — Shell spiral, turreted; aperture 

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

 generally expanded in the adult. Operculum horny and 

 spiral. The Cerithiadm are exclusively confined to the 



Fig. 397.— A, ,Si(/(')c?!(s dathratus — Eocene; b, Naticopsis pLicistria — Carboniferous (after. 

 M'Coy) ; c, Narica Genevensis — Cretaceous (after Pictet) ; d, Pyramidella Ueviuscida — Pliocene 

 (after Searles Wood) ; e, Loxoncma rKj/r/era— Carboniferous (after Pliillips) ; r, EiMma vagans— 

 Jurassic (after Morris and Lycett) ; g, Chevinitzia internodula— 'Eocene (after Searles Wood). 



Secondary, Tertiary, and Eecent periods, and are represented 

 in the Tertiary rocks by a vast number of forms. The 

 most important fossil forms belong to the genera Ceritliium, 

 Potamides, Ncrincea, and Apo7Thais, of which Nerincea is 

 extinct, and is exclusively confined to the Secondary period. 



For all practical purposes Cerithium and Potamides may 

 be considered to2;ether, as no strict line of demarcation can 

 be drawn between the fossil forms. In both, the shell is 

 turreted and many - whorled (fig. 398), with or without 

 varices. 



The aperture of the shell is small, with a tortuous an- 

 terior canal, and an expanded outer lip. Most of the living 

 forms are inhabitants 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. 



VOL. II. B 



