GASTEROPODA. 



255 



or lacustrine in their origin. The three chief or only living 

 genera are, Pallid ina, Valvata, and Ampullaria. 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. The existence of Ampullaria 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. NERITID^E. Shell thick, globular, with a very 

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

 outer lip acute. Operculum shelly, sub-spiral. The Neritida 

 are not known as occurring in the Palaeozoic Rocks, but are 

 found from the Jurassic period onwards, attaining their maxi- 

 mum at the present day. 



In the genus Nerita (fig. 219) the shell is thick, with a broad 

 columella, the inner edge of which is straight and toothed. 



Fig. 219. Nerita Schemidelliana, Eocene Tertiary. 



The outer lip is thickened and often denticulated internally. 

 The true Nerites are inhabitants of warm seas ; and they date 

 in past time from the Lias. The nearly-allied genus Neritina 

 includes the so-called " fresh- water Nerites," which agree in 

 most characters with Nerita, but inhabit fresh or brackish 

 waters. The fossil species commence in the Eocene Tertiary. 

 Lastly, the genus Pileolus comprises small limpet-shaped shells, 

 with a semi-lunar aperture below. The only fossil species are 

 from the Lower Oolites (Great Oolite). 



FAM. 15. TURBINID.*. Shell turbinated (top-shaped) or 

 pyramidal, nacreous (i.e. pearly) inside. Operculum horny and 

 multi-spiral, or calcareous and pauci-spiral. The family of the 

 TurbinidcB has a very high antiquity, dating from the Lower 

 Silurian ; but many of the older shells referred to this family 

 are of more or less doubtful affinities. The most important 

 fossil genera are Turbo, Trochus, and Euomphalus. 



In the genus Turbo (hg. 220) the shell is turbinated, with a 



