GASTEROPODA. 



249 



have a short spire and wide aperture, with an expanded and 

 flattened inner lip. They commence in the Miocene Tertiary. 

 The Helmet-shells (Cassis) begin in the Eocene, and are dis- 

 tinguished by their short spire, large body-whorl, long aperture, 

 recurved canal, and expanded inner lip. Lastly, the Olives 

 (Oliva) and Rice-shells (Olivella) are characterised by their 

 cylindrical polished shell, with a short spire, a long narrow 

 aperture, notched in front, and obliquely-striated columella. 

 The living Olives are tropical and sub-tropical in their dis- 

 tribution, and the fossil species commence in the Eocene 

 Tertiary. 



FAM. 4. CONID.E. Shell inversely conical, with a long nar- 

 row aperture, the outer lip notched at or near the suture. The 

 Conidce. commence in the Cretaceous Rocks, abound in the 

 Tertiaries, and attain their maximum at the present day. 



The true Cones form the genus C0nus, and are distinguished 

 by their short spire and regularly conical 

 shell, of which the outer lip is notched 

 near the suture. The Cones are repre- 

 sented in the Chalk, but are mainly Ter- 

 tiary and Recent. The genus Pleurotoma 

 is distinguished by a spindle-shaped shell, 

 with a long spire, the outer lip having 

 a deep slit near the suture. The genus 

 commences in the Chalk, and has an 

 enormous development in the Tertiaries, 

 from which nearly three hundred species 

 are known. The maximum, however, is 

 attained in existing seas, in which there 

 are very numerous species. 



FAM. 5. VOLUTION. Shell turreted or 

 convolute, the aperture notched in front ; 

 the Qolumella obliquely plaited. No 

 operculum. Foot very large ; mantle 

 often reflected over the shell. The living 

 members of the Volutida are chiefly in- 

 habitants of warm seas, and are often 

 remarkable for their brilliant colours. 

 The family does not appear to have ex- 

 isted till towards the later portion of the 

 Cretaceous period ; but it is abundantly 

 represented in the Tertiaries, and attains 

 its maximum in existing seas. The most 

 important genera are Voluta and Mitra. 



The true Volutes form the genus Voluta (fig. 211), charac- 



Fig. an. Valuta elongata. 

 Chalk. 



