242 PATELLID^E. 



Genus II. HEL'CION * De Montfort. PL V. f. 4. 



Body convex : mantle fringed at its edge with cirri, which 

 are alternately long and short : tentacles rather long : eyes pro- 

 minent : tongue shorter than in Patella : gills not so numerous 

 as in that genus, and forming a shorter plume, which is in- 

 terrupted over the head : foot thick, of a cellular texture. 



Shell cap-shaped, having in its embryonic state a slightly 

 twisted apex ; crown never prominent, but inflected towards 

 the anterior end, and placed near the margin — especially in the 

 young, where it is almost terminal : scars slight and indistinct. 



Besides the differences in the arrangement of the 

 pallial cirri, and in the shorter branchial plume, the 

 shell may always be known from that of Patella by its 

 shape being semioval instead of resembling a peaked 

 hat ; the crown is incurved, and the apex nearly ter- 

 minal in the present genus. The fry is not spiral, as 

 in Patella. The shell of Helcion is also usually a 

 thinner shell, with an opalescent hue ; and in the only 

 species that we possess the surface is smooth, or never 

 distinctlv ribbed. Helcion is found on Laminarias and 

 sea-weeds of a similar kind, which constitute its food ; 

 and it is therefore sublittoral in its habits. The species 

 are few, but have an extensive range, including Europe, 

 West and South Africa, Cape Horn, and Australia. 



It is the genus Nacella of Schumacher, Patina of 

 Leach, and partly Calyptra of Klein. 



Helcion pellu'cidum f, Linne. 



Patella pellucida, Linn. S. N. p. 1260; F. & H. ii. p. 429, pi. ki. f. 3, 4, 

 and (animal) pi. A A. f. 1. 



Body creamcolour, with a slight tinge of brown in front : 

 mantle often bordered by a grey or leadcoloured line, and 

 fringed with from 30 to 65 fine white cirri, half of which are 

 more than twice as long as the intermediate ones : head trans- 



* A breast-collar. f Transparent. 



