GASTEROPODA. 25 



Fam. 13. Paludinidje. — Shell conical or globular; aper- 

 ture rounded and entire ; operculum horny or shelly. The 

 Pcdudinidm are essentially inhabitants of fresh water ; though 

 they sometimes live in brackish, or even in salt water. As a 

 matter of course, therefore, they are chiefly, if not exclusively, 

 found as fossils in deposits which are believed to be fluviatile 

 or lacustrine in their origin. The three chief living genera 

 are, Paludina (fig. 411, b), Vcdvcda (fig. 411, c), and Ampid- 



Fig. 411. — A, Rissoa su'pracostata — Pliocene, enlarged six times ; b, Paludina lenta — Plio- 

 cene ; c, Valvattt piscinalis, viewed from in front and from below — Pliocene. (After Searles 

 Wood.) 



laria. The two former date from the Cretaceous period, the 

 first possibly from the Jurassic, and both abound in the 

 Wealden and in many Tertiary deposits. Bithynia resembles 

 Pcdudina, but the operculum is shelly. Valvcda may be 

 top-shaped or discoidal, but the shell is umbilicated, and the 

 peristome is entire. The existence of Ampidlarice in a fossil 

 state is attended with considerable uncertainty, chiefly from 

 the great difficulty, or impossibility, of separating them from 

 species of the marine genus Natica. 



Fam. 14. Nefjtid^. — Shell thick, globular, with a very 

 small spire; aperture semi -lunate, its columellar side ex- 

 panded ; outer lip acute. Operculum shelly, sub-spiral. The 

 Neritidce are not known as occurring in the Palseozoic rocks, 

 but are found from the Jurassic period onwards, attaining 

 their maximum at the present day. 



In the genus Nerita (fig. 412) the shell is thick, with a 

 broad columella, the inner edge of which is straight and 

 toothed. The outer lip is thickened and often denticulated 

 internally. The true Nerites are inhabitants of warm seas ; 



