3 

 TUi: niMMoX TJMPET (PafJ/,, ,ul,i<ita). 



GrE^ERAL DESCRIPTION. 



Tho Limpet l)oloiio's to tlio asymmetiical class of 

 Mnllusca (Gastrojxida), ami, thoug-h it lias lost both 

 ctenidia, is referred to the order Prosohranchia, the 

 members of which have the ctenidiiim (or ctenidia) 

 anterior to the heart. Among- the Prosobranchia its place 

 is in the g-ronp 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 their 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. vidgafa has its shell substance greyish or yellowish, 

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

 the radiating ribs vaiy 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 llanley state that 

 Clark found the pallial tentacles shorter and thicker and 

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

 is fairly abundant but is locally distributed. It is usually 

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

 be tougher than /-*. vutgata. Jeffreys considers P. 

 athletica to be merely a variety, and he enumerates three 

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

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

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



