42 THE NAUTILUS. 



is to C. fornicata, as has been already noticed by 



jNIr. Pilsbry. In both of them the beak projects 



almost directly forward and stands clear above 



the margin, while in C. fornicata it is very much 



to one side and rests directly on the margin. This 



Crepiduia giauca. scenis to be caused by the concentric additions to 



the shell being very unequal — i. e. wider on one side than on the 



other — in the latter species. The outline of the posterior margin of 



the "deck " or septum in C. fornicata is also different. 



The difference between C. giauca and convexa is entirely one of 

 shape and is apparently due primarily to the character of the surface 

 on which they live, as C. giauca occurs on large shells where the 

 curvature is slight and C. convexa on small, very convex shells such 

 as Nassa obsoleta, where considerable curvature is necessary to enable 

 the Crepiduia to bring the entire margin of its shell in contact with 

 the surface of its host. 



Whether this difference of shape is to be regarded as a specific or 

 subspecific character is a question dependent entirely upon the 

 extent to which this adaptation to two styles of surface has 

 proceeded. If shells of intermediate form occur, then C. 

 coniea;a is merely a subspecies. Among those which I have 

 thus far examined I have seen no intermediates. 



The fact that we have at least two distinct convex c. convexa. 

 Crepidulas upon the New Jersey coast naturally suggests the pos- 

 sibility that we may have had at one time, two concave species ; one 

 developed from the same stock as C. fornicata, as already described, 

 and the other in a similar manner from the G. giauca stock. The 

 similar environment and degenerate nature of "inside growers" 

 would tend to obliterate the original specific differences, so that the 

 two might easily have become merged into what we know as C. 

 plana. 



Anatomical investigations might throw additional light upon 

 these questions, and a careful study of the genus will well repay 

 anyone who has the requisite material at hand. 



THE SHELL BEARING MOLLUSCA OF MICHIGAN.* 



BY BRYANT WALKER. 



U. anodontoides Lea. Cited by De Camp from Monroe County. 

 Also by Call. 



* The first installment (land mollusks) of Mr. Walker's catalogue of Michigan 

 niollusks will be found in the June Nautilus; the second, aquatic gastropods, in 

 the July number. 



