PROSOBRANCHIATA. 



791 



between the tropics. A single species has been found in the 

 highest Jurassic beds in Sicily (Zittel), but with this exception the 



Fig. 696. — Rostellaria amfila, reduced one-third. Eocene Tertiary. 



fossil species date from the Cretaceous period, and abound in 

 the Tertiaries. 



The shell of the Cowries in the young state is furnished with a 

 prominent spire, and has a thin outer lip. In 

 the adult state (fig. 697) the spire is com- 

 pletely concealed within the shell, the entire 

 surface is generally covered with shining 

 enamel, the inner lip is crenulated, and the 

 outer lip is thickened, inflected, and crenu- 

 lated. The small Cowries of which the com- 

 mon Cyprcza {Trivia) Europcea is the type, 

 are not known to occur in the Secondary 

 rocks, but have a few Tertiary representatives. 

 They are distinguished from the ordinary 

 Cowries by the fact that the upper surface is 

 adorned with transverse ribs. The genus 

 Ovidum is closely related to Cyprcea, but the 

 aperture is drawn out anteriorly and posteriorly, and the inner lip 

 is smooth. The genus dates from the Eocene, and possesses a 

 number of existing representatives. Erato, again, ranging from 



Fig. 697. — Cyfircea elegans. 

 Eocene Tertiary. 



