814 



DIVISIONS OF THE GASTROPODA. 



habitants of fresh water, and they are found in fluviatile and lacus- 

 trine deposits. They commence in the Jurassic period, members 

 of this family having been described from the 

 Lias (?) and from the Purbeck beds (Upper Jur- 

 assic). It is not, however, until we reach the 

 base of the Cretaceous system (Weald Clay) that 

 these forms appear in any abundance. 



The genus Limncea (fig. 728) includes the so- 

 called " Pond-snails," characterised by their thin, 

 spiral, elongated shells, with a large body-whorl 

 and an obliquely-twisted columella. The species 

 of this genus commence in the Upper Jurassic 

 (Purbeck Beds), and they are abundantly repre- 

 sented in the Tertiary series. 



In the genus Physa (fig. 729) the shell is left-handed (" sinistral "), 

 ovate, thin, and polished, with the aperture rounded in front. Wal- 

 cott has described a species of Physa as occurring in the Lower 

 Carboniferous rocks of Nevada. With this exception, the oldest 

 species of the genus occur in the Purbeck beds (Upper Jurassic) 

 and Wealden (Lower Cretaceous). Most of the fossil species, how- 

 ever, belong to the Tertiary period, and the genus attains its maxi- 

 mum at the present day. 



The genus Planorbis (fig. 730, a and b) comprises a number of 

 well-known fresh-water shells, in which the shell is discoidal and 



Fig. 728. — Li7nncea 

 fiyramidalis. Eocene 



Fig. 729.— Physa colum- 

 nan's. Eocene. 



^> 



Fig. 730. — a, Planorbis C07nplanatus, viewed from below 

 and in front — Pliocene and Recent ; B, Planorbis discus — 

 Eocene — viewed from above and in front, reduced one-half; 

 c, Ancylus Matheroni — Tertiary — viewed sideways and 

 from above, the latter figure enlarged. 



many-whorled, the aperture crescentic, and the lip thin. The fossil 

 species of this genus date from the Lias (?), but are not plentiful 

 except in the Tertiary deposits, from which numerous forms have 

 been obtained. 



Lastly, the genus Ancylus (fig. 730, c) comprises the so-called 

 " River-limpets," distinguished by their thin limpet-shaped shell, the 

 apex of which is approximated to the hinder margin. The fossil 

 species are few in number, and the earliest forms appear in the 

 Miocene deposits. 



