268 MOLLUSCA. 



elongated shells, with a large body-whorl and an obliquely- 

 twisted columella. The species of this genus commence in 

 the Wealden (Lower Cretaceous) perhaps in the Upper 

 Oolites and are abundantly represented throughout the Ter- 

 tiary series. 



In the genus Physa (fig. 239) the shell is left-handed (" sinis- 

 tral "), ovate, thin, and polished, with the aperture rounded in 

 front. Species of this genus have been indi- 

 cated as occurring in the Purbeck beds (Up- 

 per Oolites) and Wealden (Cretaceous). 

 Most of the fossil species, however, belong to 

 the Tertiary period, and the genus attains its 

 maximum at the present day. 



The genus Ancylus comprises the so-called 

 " River-Limpets," at once distinguished by 

 their thin, conical, limpet-shaped shells. A few 

 fossil species are known, chiefly, if not exclu- 

 Fi g . 2 39; rhysa. sively, confined to the Tertiary period. 



The genus Planorbis comprises a number 

 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^E : Shell spiral, with a horny epidermis ; 

 aperture elongated and denticulated. The species of this 

 family inhabit salt marshes and places overflowed by the sea. 

 They are of little importance as fossils, dating from the Eocene 

 Tertiary. 



ORDER V. OPERCULATA : Shell furnished with an oper- 

 culum. 



FAM. 5. CYCLOSTOMID^ : Shell spiral, rarely elongated, 

 often depressed. Aperture nearly circular. 

 Operculum spiral. The genus Cydostoma 

 (fig. 240) includes almost all the fossil 

 species of this family, and dates from the 

 Eocene Tertiary. All the members of this 

 family are terrestrial in their habits, and 

 they are of small importance as fossils. 

 FAM. 6. ACICULID^ : Shell elongated, 



Fig. *4p.Cyciosto>na. cylindrical ; operculum thin and sub- 

 ArnoudU. Eocene Ter- S pi ra l. A species of Aciciila has been in- 

 dicated as occurring in the Pliocene Terti- 

 ary ; but the family is otherwise unrepresented by fossil forms. 



