MANUAL OF THE MOLLUSCA. 



Distribution, 29 species. Cretaceous; India. Eocene. Prance 

 Ketherlands. 



ExiSTOMA, Piette, 1855. 



Type, E. tuberculosa, Piette. 



Shell in the young state resembling Cerithium ; in the adult, 

 tho margins of the aperture are much expanded and posteriorly 

 united by an indistinct canal ; canal elongated. 



Fossil, 2 species. Great Oolite. Ardennes. 



ExELissA, Piette, 1861. 



Etymology, exelisso, to unfold. 



Synonym, Kilyertia, Lycett, 1863. 



Type, Cerithium strangulatum, D'Archiac. 



Shell small, elongated, subcylindrical, somewhat pupeeform, 

 many whorled, perpendicularly costated, tuberculated or spined; 

 last whorl cylindrical, contracted at the base, with a tendency 

 to separate from the y,xis ; aperture orbicular, entire, the lips 

 elevated, produced, and slightly thickened ; columella soHd. 



Fossil, 14 species. Mid. Lias— Kimmeridge Clay. England 

 and Erance. The sheUy freestone of the Inferior OoHte, G-lou- 

 cestershire, contains some undescribed species. Cretaceous, 1 

 sp. ? India. 



EiBULA, Piette, 1857. 



Example, Turritella Eoissyi, D'Archiac. 



Shell elongated, columella straight, with a rudimentary 

 groove near the base ; outer lip arched, slightly notched at the 

 suture ; base of the aperture forming a slight canal, or rounded 

 and entire, depending upon the exact period of growth at which 

 the animal perished. 



The species of this genus possess characters intermediate and^ 

 approximating them to Turritella and to Cerithium. 



Fossil, 21 species. Triassic— Cretaceous. Europe, India. 



CnYPTOPLOcus, Pictet and Campiche, 1854. 



Etymology, cryptos, hidden ; ploce, a plait. 



Example, Nerinsea monilifera, D'Orb. 



Shell, as in Nerincea, without columella and labial plaits ; one 

 plait on the posterior face of the aperture, a disposition verj- 

 analogous to that in some Cerithiums, such as 0. nodulosum; 

 aperture rounds ^ not channeled in front ; umbilicated or im- 

 perforate. 

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