EXTERNAL FORM OF SHELLS. 419 



former having been founded on specimens taken from cavi- 

 ties in the root, and the latter on individuals obtained from 

 the flattened frond of the Fucus on which the species usually 

 takes up its abode : it is, indeed, by no means rare to find 

 specimens in which the animal has moved from one of these 

 positions to the other, and in such cases the apex of the 

 shell represents P. caerulea and the base P. pellucida, or 

 vice versa. The same change takes place with regard to 

 P. miniata and P. compressa. I have in my collection a 

 specimen of this latter shell which is P. miniata at the top, 

 it having in its youth lived on the frond of a large Cape 

 Fucus ; it afterwards removed to the stem, and became com- 

 pressed, and consequently is in this part the P. compressa ; 

 but by some accident it was again induced to change its situ- 

 ation, and, removing to a flat surface, the edge of the mouth 

 expanded, and it became a second time P. miniata, or per- 

 haps what may be called by some authors P. saccharina, as 

 this also appears to be a conical variety of the same species. 

 Lamarck has described a similar specimen ; and Mr. Sower- 

 by, in his Genera of Shells, has figured an example of this 

 species, showing the two states. In like manner the Crepi- 

 dula porcellana, when applied to a flat surface, has an ex- 

 panded base and a flattened inner lip ; but when adherent 

 to a convex, body, such as the stem of the sea-weed, or (as 

 frequently happens) to the back of another shell of the 

 same species, the animal being pressed into the cavity, the 

 inner lip becomes concave, and the sides of the aperture are 

 contracted : in this state the shell is called by most authors 

 C. fornicata. 



When the shells of this family are adherent to irregular 

 surfaces, they adapt their margins to the inequalities with 

 which they meet. I have several specimens of Patella from 

 the coast of Devonshire, having one or more processes on 

 their sides, which fitted into holes in the rock to which I 

 found them attached ; and such changes are the more re- 

 markable, as some specimens are seen constantly moving 

 from place to place, whilst others appear to remain for a 

 long time fixed in one spot, and even those that are thus 

 stationary in the young state constantly elevate the margins 

 of their shells when the tide is low. I have also a specimen 

 of Siphonaria gigas exhibiting in a great degree a similar 

 adaptation of its edges to the form of the rock on which it 

 grew. 



The substances to which attached shells become adherent, 

 besides altering their general form, often change the cha- 

 racter of their surfaces ; thus, when fixed to ribbed shells, 



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