230 PATELLID^. 



as lias been stated by some writers. Wben first formed 

 they are either spiral or else eccentrically twisted ; the 

 spire or twist is worn away in the course of growth. 

 There being no communication between the mantle and 

 the apex of the shell, the latter cannot be absorbed by the 

 animal. The sexes are separate. Those kinds which inha- 

 bit the Httoral and laminarian zones are phytophagous ; 

 and the others, which inhabit the coralline and deep-sea 

 zones, are probably zoophagous. In Loven^s scheme of 

 the dentition in univalve mollusca the rhachis or central 

 plate in Patella and Helcion has six teeth, and each of 

 the pleurse or side plates three teeth ; in Tectura the 

 rhachis has from four to six teeth, and the pleurae have 

 none ; in Lepeta the rhachis has only a single tooth, 

 and each of the pleurae two. Such differences may in- 

 dicate the nature of the food ; the first three genera are 

 known to live on sea-weeds, while the last (as well as 

 Propilidium) cannot derive their subsistence from any 

 vegetable matter except diatoms. 



Genus I. PATE'LLA^, Lister. PI. V. f. 3. 



Body convex : mantle fringed at its edge with cirri of irre- 

 gular lengths: tentacles rather short: eijes prominent: gills 

 numerous and closely packed, lying between the mantle and 

 the foot, and only interrupted on the right-hand side : foot 

 thick and muscular. 



Shell conical, more or less convex, furnished with ribs that 

 radiate from the crown, having in its embryonic state a com- 

 pletely spnal apex ; crown prominent, eccentric but not 

 very much on one side ; the attachment of the mantle to the 

 shell is exhibited in the middle (between the crown and the 

 margin) as a ring-like scar. 



The Xe7ra9 of the Greeks, with whom it appears to 

 have been rather a favourite article of food. In the 



* A small pan. 



