( 32 ) 



DIVISION XV. HEMICYCLOSTOMATA. 



GENERA. 



NeritA, Lin. 



Semi-globulous ; mouth Columella in a straight 

 semi-circular or semi-el- line, 

 liptical, closed entirely by 

 an operculum ; spire al- 

 most effaced. 



a. Natica, (4) Lam. 



Operculum homy. Columella umbilicate. Marine. 

 PI. 9, fig. 24. V. Animal, p. 67. 



b. Nerita, (2) Lam. 



Operculum stony, Columella umbilicate, Marine. 

 shell thick. PI. 9 5 fig. dentated. V. Animal, p. 67. 



20. 



e. Neritina, Lam. 



Operculum horny, Columella rarely den- Fluviatic. 

 shell thin. PI. 9, fig. tated. (3) V. Animal, p. 67. 



21. 



(1) In the fossil state, in the London Clay and Crag Marl; at Grignon. 



(2) In the fossil state, at Rethcuil, Courtagnon, Houdan, Grignon. 



(3) Those with the columella dentated form the genus Clitlwn of Montfort, whose 

 Clitlwn corona, pi. 9, fig. 23 ; differs from the Clithon coronata of Leach. 



