26 



GASTEROPODA. 



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, and have a comparatively thin smooth 



il2.— Nerita ScliuiihMlh 



Ericeiie Tertiary. 



shell. The fossil species commence in the Eocene Tertiary. 

 The Jurassic genus Neritoma has a thick ventricose shell, 

 with a notch in the middle of the outer lip. Ncritopsis, 

 again, commences in the early portion of the Secondary 

 period, and still survives. Its shell is distinguished by the 

 possession of a single notch in the middle of the inner lip. 

 In connection with tliis genus we must mention the curious 

 Jurassic fossils which have been described under the name 

 of Pdtarion. These are oval or nearly circular calcareous 

 plates, concave above and flattened below ; and they have 

 generally been regarded as the mandibles of Tetrabranchiate 

 Cephalopods. Recent researches, however, seem to show 

 that these singular bodies are really the opercula of species 

 of Ncritopsis. 



Lastly, the genus Pilcolus comprises small limpet-shaped 

 shells, with a semi-lunar aperture below. The only known 

 fossil species are from the Lower Oolites (Great Oolite). 



Fam. 15. TuPuBiNiD^. — 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 TurUnidce has a high antiquity, the forms ascribed to 

 it dating from the Lower Silurian ; but many of the older 

 shells referred to this family are of more or less doubtful 

 affinities. 



In the genus Ticrho (fig. 413) the shell is turbinated, with 



