116 SYSTEMATIC ARRANGEMENT 



lateral teeth, first produced on the outside, the tip sharp-pointed and 

 serrated on both sides, second and third somewhat similar, claw-like, 

 the former more curved, the latter simple. 



The foot of Crepidula is oblong, nearly the size and form of the 

 concavity of the under side of the shell to the edge of the inner lip ; 

 the rest of the chamber is occupied by the dilated head, which is 

 rather expanded on the sides. The tentacles are linear, with the eyes 

 on small tubercles on the outer side of the base, and the forehead is 

 notched. 



The species of these attached shells are very difficult to distin- 

 guish, from the various forms they assume under different external 

 circumstances. From the fact of each species assuming, under the 

 same circumstances, a peculiar form, it has been thought that they 

 must all be varieties of the same species ; but I am inclined to think 

 that most probably the species of each distinct locality are different, 

 though each perhaps offering the same variations : thus, the Crepi- 

 dulce of the Mediterranean, of Peru, and the east coast of North 

 America, though they may each present the four common varieties, 

 may be four distinct species, the geographic character being perhaps 

 the best distinctive mark. 



In Sicily, Crepidula fornicata is always fixed on the outside and 

 C. unguiformis on the inside of shells ; the former is streaked with 

 brown, and generally covered with Vermeti, Flustrce, and other para- 

 sites ; sometimes the edge of the shell is sinuous, from the irregular 

 form of the body on which it was living, and hence called C. sinuosa 

 by Turton. These characters have been used to distinguish the 

 species, instead of proving they are only one, in different situations. 



Crepidula fornicata, which is convex and coloured, M. Philippi 

 (Moll. Sicil. i. 126) observes, in Sicily is always on the outer surface 

 of the shell, while C. unguiformis, which is flat, white and smooth, is 

 always attached to the internal surface of the shell ; having collected 

 1 1 6 specimens in that situation, he gives this difference of habitat as 

 a proof of their being distinct species, while I am inclined to regard 

 it as a proof that the difference in the forms and colour depend on 

 the part of the shell to which they happen to be attached. 



Many of these flat, white specimens of Crepidula unguiformis have 

 near the tip a more convex part, showing the form of the shell in its 

 younger state, and this part is often streaked with brown like the 

 convex variety. These specimens, combining the two varieties in 

 the same individual, also prove that these animals sometimes do 

 not take up the habitation on the inside of the shell, until they attain 

 a certain size, as when they do so they suddenly change the form 

 and colour of their shells. 



On the other hand, Mr. Stimpson states that Crepidula fornicata 

 sometimes occurs with C. unguiformis in the apertures of univalves, 

 where it preserves its colour, convexity and other characters. — Shells 

 of New Eng. 30. [I have never seen them in that situation.] 



Crepidula fornicata varies in shape according to the body on 

 which it rests ; four or five different ages arc frequently found riding 

 upon each other. When growing on Pecten concentricus, it is found 



