THE COMMOX LTMrET (Patella nihiala). 

 General Description. 



The Limpei boloiig-s to tlip asynimetrioal class of 

 ^rnllusoa {Gastropoda), and, tliougli it has lost hoth 

 ftenidia, is referred lo the order ProsO'branchia, the 

 members of which have tlie ctenidiiim (or ctenidia) 

 anterior to tlie heart. Among the Prosobranchia its place 

 is in the group of Docoglossa, while the absence of ctenidia 

 and presence of a compensating circlet of pallial gills show 

 that it belongs to the sub-group Cyclobranchiata. 



Limpet shells are so simple and tbeir few characters 

 are so subject to variation, that it is not easy to classify 

 the British forms into varieties and species. Of these, 

 however, two are generally recognised : — 



P. vulgata has its shell substance greyish or yellowish, 

 but never white. The margin is fairly simple, and, though 

 the radiating ribs vary a good deal, they are not usually 

 very strongly developed. Aged shells have their vertices 

 more central than those of young ones. 



P. athletica has its shell substance white, and possesses 

 numerous elevated ribs with regular series of projecting 

 scales, the interstices being stained with brown. The 

 shell is usually depressed. Forbes and Ilanley state that 

 riark found the pallial tentacles shorter and thicker and 

 the pallial gills longer than in P. vulgata. This species 

 is fairly abundant but is locally distributed. It is usually 

 found far down the intertldal zone, and its flesh is said to 

 be tougher than P. vulgata. Jeffreys considers P. 

 athletica to be merely a variety, and he enumerates three 

 other varieties : — P. elevata — small, round, high ; P. jnda 

 — small, thin, alternate ribs reddish and dark blue ; P. 

 iiitfrmedia — smaller, flatter, and oval, finer ribs, orange 



