PROSOBRANCHIATA. 



797 



■jog. — Typhis tubifer. Eocene 

 Tertiary. 



whorl. The aperture is notched in front or produced into a short 

 canal, and the columella is ob- 

 liquely plaited. There is usu- 

 ally no operculum, the foot is 

 very large, and the mantle is 

 often reflected over the shell. 

 The living members of the 

 Volutidce are chiefly inhabi- 

 tants of warm seas, and are 

 often remarkable for their 

 brilliant colours. The family 

 does not appear to have exist- 

 ed till towards the later portion 

 of the Cretaceous period ; but 

 it is abundantly represented in the Tertiaries, and attains its maxi- 

 mum in existing seas. The most important genera are Voluta, Vol- 

 utomitra, Mitra, and Marginella. 



The true Volutes form the genus Voluta 

 (fig. 710), characterised by the short spire, 

 large, deeply-notched aperture, and columella 

 with several plaits. A large number of recent, 

 and still more numerous fossil, forms of Voluta 

 are known, the earliest types appearing in the 

 Cretaceous rocks. The genus has been 

 divided into a number of sub-genera, one of 

 the most important of which is Volutilithes, 

 represented by numerous forms in the Creta- 

 ceous and older Tertiary deposits. 



Mitra, Volutomitra, and Turricula, are 

 hardly distinguishable as fossils, all of them 

 possessing a spindle-shaped shell, with a long 

 spire and a small aperture, an anterior notch 

 or short canal being present, and the colu- 

 mella being obliquely plaited. The typical 

 Mitrce have the outer lip thickened and 

 smooth within, and are characteristic of the 

 Tertiary and Recent periods. In Turricula 

 the outer lip is striated internally, and the 

 surface is transversely ribbed. Forms of 

 this type range from the Cretaceous rocks 

 to the present day. In Volutomitra (Ter- 

 tiary and Recent) the outer lip is thin and 

 simple. 



In Marginella, lastly, the shell is smooth, 

 with a short or concealed spire ; and the aperture is truncated 



Fig. 71 



— Voluta elongata. 



Chalk. 



