1898.] NATURAL SCIENCES OF PHILADELPHIA. 435 



ENVIRONMENTAL AND SEXUAL DIMORPHISM IN CREPIDULA. 

 BY EDWIN G. CONKLIN, PH. D. 



I. The genus Crepidula Lam. is represented on our North Atlan- 

 tic Coast by at least three species, viz. : C. fornicata Lam., C. plana 

 Say, and C. convexa Say, while the species C. adunca Sby. and C. 

 navicelloides are abundant on the Pacific Coast of the United States. 



All these species are more or less completely sedentary, and they 

 are usually, though not invariably, carried about by other animals 

 with whom they are messmates and upon whom they are securely 

 fixed. All the larger species of Crepidula are immovably fixed to 

 one spot, e. g., C. fornicata, C. plana, C. navicelloides, while the 

 smaller species C. convexa and C. adunca are able to move about to 

 a limited extent. 



Among these smaller forms the characters of the shell are fairly 

 constant, but among the larger forms it is well-nigh impossible to 

 tell what the normal or usual form is ; this is especially the case 

 with C. plana, where it is a rare thing to find two shells alike. 



Even among the smaller species there are marked local varieties 

 depending upon the immediate environment, e. g., C convexa as 

 found on Illyonassa and Littorina shells is deeply convex and very 

 darkly pigmented. On oyster shells it is very much flatter and 

 lighter in color, and is frequently mottled as shown in Plate XXI, 

 Row 2. This local variety has been considered a distinct species, 

 viz. : C. glauca Say ; its anatomical and embryological characters 

 show, however, that it is not specifically distinct from C. convexa. 

 The same is true of Lea's species, C. acuta, which is merely a local 

 form of C. convexa. 



Among the larger and more sedentary species, C. fornicata and C. 

 plana, the most remarkable differences in the shape of the shells may 

 be observed due to the character of the surface upon which they are 

 attached. " Upon a smooth, plane surface, the shell is regular and 

 unusually broad and flat ; on a convex surface it is deep and highly 

 arched ; on a concave surface it is concave ; on a twisted surface, such 

 as the columella of Neverita, it is twisted ; on an irregular surface, 

 such as a rough stone, it is irregular ; if pressed upon from the sides, 



