Tusk-shells^ Mail-shells, and Limpets 193 



The genus Helcion differs from Patelht in the fact 

 that the frill of gill-plates arising from the left side 

 of the neck is not continued completely around the 

 animal, but ends in front of the head. In early life 

 the shell of the Blue - rayed Limpet {Helcion "pel- 

 liicidiiin) is oval, thin, semi-transparent, 

 rounded above, with the slight beak 

 above the front extremity. The colour 

 varies from ochreous to olive, and from 

 the beak thin broken lines of vivid blue 

 run to the margins. The number of these lines 

 varies greatly, but, as a rule, they are about 7, 

 though they may be 40. The animal is cream 

 coloured, tinged with brown ; mantle fringed with 

 long and short filaments ; tentacles slender, eyes 

 small. I have already alluded to the changed 

 appearance of the shell when the animal becomes 

 old (page 30). The young Helcion lives upon the 

 fronds of the larger seaweeds, where it is not very 

 conspicuous, but in later life it descends 

 the rounded stalks, thickening its shell, 

 compressing the sides and fashioning 

 the mouth so that it fits the rounded 

 "oTd'XlT ^"rf'ico accurately. By this time the 

 blue lines have vanished, as being a 

 source of danger rather than security in the new 

 situation, and the shell has no resemblance to its 

 former self. The animal cuts out a cavity in the 

 stalk, which it resembles in colour, and there it 

 remains embedded. 



The Slit - limpets (Fissurellidce) form a family 

 remarkable from the fact that their limpet - like 

 shells have a slit which serves the purpose of an 



