HETEROPODA AND PULMONIFERA. 



45 



the Wealden (Lower Cretaceous)— perhaps in the Upper 

 OoKtes — and are abundantly represented throughout the 

 Tertiary series. 



In the genus Physa (fig. 436) the shell is left-handed 

 (" sinistral "), ovate, thin, and polished, with the aperture 

 rounded in front. Species of tliis genus have been indi- 

 cated as occurring in the Purbeck beds (Upper Oolites) and 

 Wealden (Cretaceous). Most of the fossil species, however, 

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

 mum at the present day. 



The genus Ancylus (fig. 437, c) comprises the so-called 

 " Eiver- limpets," at once distinguished by their thin, coni- 

 cal, limpet-shaped shells. A few fossil species are known, 

 chiefly, if not exclusively, confined to the Tertiary period. 

 The Valenciennesia, of the Tertiary, resembles a gigantic 

 Ancylus, but the apex is much incurved, and an internal 

 sulcus and corresponding superficial fold extend from the 

 apex to the right margin, and exist in a less developed form 

 on the left side. 



Fig. 436 — Phy?a colum- 

 nans. Eocene. 



Fig. 437. — A, PkmorMs covvplanatiis, viewed from below 

 and in front — Pliocene and Eecent ; b, Planorbis discus 

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

 half; c, Ancylus Matheroni — Tertiai-y — viewed sideways 

 and from above, the latter figm-e enlarged. 



The genus Planorlis (fig. 437, A and b) comprises a num- 

 ber of well-known fresh-water shells, in which the shell is 

 discoidal and 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, whence a large number of forms has been obtained. 



Fam. 4. AURICULID^. — Shell spiral, with a horny epi- 

 dermis ; aperture elongated and denticulated. The species 

 of this family inhabit salt-marshes and places overflowed by 



