467 



FISSURELLlDiE. 



The passage from the Limpets to completely spiral uni- 

 valves, is very naturally maintained by the members of this 

 family, all of which are remarkable for some solution of 

 continuity in the shell, either a perforation or a slit in the 

 region of the vent, and with reference to the construc- 

 tion of that organ, though often spoken of by conchological 

 writers as if it were a branchial orifice. The form of 

 their shells is more or less conical, with indications of 

 a rudimentary spire at the apex, which often disappears 

 with advancing age. The animals have well develojied 

 heads, with short muzzles, and subulate tentacles, at the 

 external bases of which are the eyes placed on rudimentary 

 pedicles. Between the sides of the foot at the mantle 

 is a range of short lateral cirrhi. These characters of 

 peduucated eyes and cirrhated sides indicate a close 

 approach to the Troclndee. and Haliotidece, an indication of 

 affinity which is fully borne out by the arrangement of 

 the lingual teeth. There are two symmetrical branchial 

 plumes. 



The genera are distinguished from each other by the 

 characters of the mantle and the disposition of the vent, 

 both of which affect materially the form and characters of 

 the shell, consequently the family is composed of groups 

 equally well defined, conchologically and malacologically. 

 All its members are marine, some littoral, some inhabiting 



