"INVERTEBRATE PALEONTOLOGY. 17 



Surface only marked by obscure lines, and small, irregular, nearly obsolete 

 wrinkles of growth. 



Length and breadth of a medium -sized specimen, each about 2.40 inches ; 

 concavity oi lower valve, 0.65 inch. 



The specimens from which the above description was made out might 

 with almost equal propriety be referred to Gryphcea, and really bear some 

 i escmblance to Gryphcea vesicularis, Lam. (=z G. convexa of Say and Mor- 

 ton); but they present well-marked and constant differences from all the 

 numerous varieties of that protean species. In the first place, the lower 

 valve of our shell is never so ventricose as that of the common varieties of p. 

 ■vesicularis, and always differs in having its cardinal margin sloping from the 

 beaks instead of being extended in a right, line, and auriculate at the extrem- 

 ities. Again, its umbo is never near so gibbous, nor so distinctly incurved 

 as is usual in that species; while its upper valve is always destitute of any 

 traces of the radiating depressed lines generally seen on that of G. vesicularis. 



In the shallowness of its under valve, and the compressed form of its 

 beak, it agrees with G. mutabilis of Morton (which is also generally regarded 

 as a marked variety of G. vesicularis) ; but it differs in the other characters 

 mentioned, as G. mutabilis has the cardinal margin as straight, and its extrem- 

 ities as distinctly ariculate, as well as its smaller valve as strongly marked 

 with radiating lines, as the more convex varieties of G. vesicularis. 



Associated with the specimens presenting the characters given in the 

 foregoing description, there are many others which pass by slight shades of 

 difference into much more irregular forms. Some of these scarcely differ in 

 any other respects from those we have taken as the type of the species, than 

 in having the umbo of the lower valve a little distorted by the scar of attach- 

 ment; while others show still more and more distinct marks of attachment, 

 become more elongate or ovale in form, and have the left or anal margin near 

 the beak more and more sinuous, until they have departed widely from the 

 typical forms. It is possible that some of these may belong to one or more 

 distinct species; but I confess, after a careful study of a large number of spe- 

 cimens, that I am totally at a loss to find constant characters by which they 

 can be separated. I therefore prefer, with the information now at command, 

 to present some of the best marked of these different forms rather as vari- 

 eties than as different species. 



