CALYPTRACEA. MOLLUSCA. CREPIDULA. 159 



other. When growing upon Pecten concenlricus it is found to have 

 ribs corresponding to those of the Pecten. It is a solid shell, and the 

 diaphragm is situated near the mouth, leaving no cavity extending 

 under the beak. The manner in which one edge is pressed against 

 the side of the shell is quite characteristic. The margin of the aper- 

 ture is generally white, dotted with chestnut ; the remainder of the 

 interior is more or less brown. Sometimes the attachment of the 

 diaphragm is bordered with reddish-brown. 



CREPIDULA PLANA. 

 Shell ovate, flat, white ; apex acute, terminal ; diaphragm convex. 



FIGURE 16. 

 State Coll., No. 143. Soc. Cab., No. 821. 



Crepidula plana, SAT; Journ. dead. Nat. Sc., ii. 226. Amtr. Conch., pi. 44. 



Shell ovate, flat, or as often a little concave or convex, thin, 

 transparent, white, wrinkled with concentric lines of growth ; 

 apex minute, pointed, turning a little to one side, and constituting 

 the extreme termination of the shell ; the other extremity broader, 

 and regularly rounded ; interior white, of a brilliant polish, and 

 iridescent ; diaphragm less than half the length of the shell, con- 

 vex, rising to a level with the margin, free edge for the most part 

 straight, but having a projecting angle near one side. Length 1| 

 inch, breadth T \ inch. 



Found in the aperture of other shells. 



This species has also been observed through a wide region. Mr. 

 Say noted it as far south as Florida. It does not, however, frequent 

 localities north of us. It is still regarded by some as a variety of 

 the C. fornicdta modified by its position. But the peculiarity of form, 

 coloration, diaphragm, and habit of living, seem to render it sufficient- 

 ly distinct. When young, it is of a more rounded form, but becomes 

 elongated by age. It is otherwise very variable in shape, conforming 

 to the position it occupies in the throat of some other shell. 



This is very likely to prove to be the C. unguiformis, Lam., (Pa- 

 tella crepidula, Lin.). In this opinion I have the concurrence of Mr. 

 Sowerby. Deshayes observes that he can hardly think that the shell 

 figured as Calyptrda unguiformis by Broderip, in " Trans. Zool. 

 Soc.," i. pi. 29, f. 4, is the shell of Linneeus. He says the shell of 



