468 FISSURELLID^. 



considerable depths. They are distributed through the 

 seas of all dimates. 



FISSURELLA, Bruguiere. 



Shell conical, usually thick and depressed, the vertex of 

 the adult truncate and perforate, and placed on the shorter, 

 which is the anterior portion of the shell ; aperture widely 

 expanded, oblong ; surface with radiating, often cancellated, 

 striae or ribs. Interior without any partition ; muscular 

 impression elongate, crescentic, interrupted in the region of 

 the head. 



Animal with a short muzzle, terminating a tumid head, 

 bearing two subulate tentacles, at the external bases of 

 which are the eyes, placed on rudimentary pedicles ; a 

 range of numerous cirrhi around the sides at the base of 

 the very large foot ; mantle produced in front, with a 

 fringe of cirrhi above its margin ; anal siphon in the form 

 of a short, truncated, membranous canal, projecting from 

 the apical aperture of the shell ; branchial plumes two. 



The fry of this genus has the vertex of the shell entire 

 and subspiral ; the perforation at that stage assumes the 

 position which we see it permanently take in Rimula and 

 Puncturella. The species o^ Fissurella are numerous, often 

 large and handsome, and mostly inhabitants of tropical 

 seas. But few are known in the fossil state. The majority 

 of living ones inhabit shallow water. They are poj)ularly 

 known as Key-hole Limpets, on account of the peculiar 

 shape of the apical orifice. 



