GASTEROPODA. 



137 



shells, almost eaten away, in consequence of which the 

 animal thickens that part. 



Philippi unites under this the following sub-genera : — 

 Neritina Lam. ; Clithon Montf. ; Theodoxus Montf. ; 

 Corona Chemn. ; Neripteron Lisson ; Clypeolum Recluz ; 

 Mitrula Menke ; and Velates Montf. 



Navicella. Lam. (Cimber Montf. ; Septaria Fer. ; 

 Catillus Humph.) — Shell oblong, convex above, with the 

 apex not spiral, but bent down to the edge ; 

 inner lip thin, and assuming the form of a jm^' \ 

 plate across one end of the aperture ; the f^^^^s 

 operculum shelly, of a square form, thin, j j ! ( ^ 

 flat, and furnished with a sharp point pro- 1^ / 1 

 ceeding from the side at one corner. (Sow.) Vl|" ji 

 Philippi says that the operculum is within 

 the animal, dividing the intestines from the 



.... . . Navicella porcellana. 



foot, which distinguishes it from Nerita. 

 Animal with a broad head, half-moon shaped, eyes on short 

 stalks at the base of the feelers, the foot large and thin 

 edged. — 18 species; also fossil. 



These mollusca are fluviatile, inhabiting the clear rivers 

 of India, the Isle of France, the Polynesian Islands, &c. ; 

 they creep on the rocks, and do not continue fixed to one 

 spot, as the Patella: the dark zigzag marks on the shell 

 are seen through the epidermis. In the Isle of France 

 they are said to be made into broth for invalids. 



Pileolus. Sow. (Tomostoma Desh.) — Shell Patella- 

 like, the apex not quite central, the inner lip is flat and 

 crenulated, and the aperture half-moon shaped ; the outer 

 lip marginated. — 2 species, fossil. 



