INVERTEBRATE PALEONTOLOGY. 45 



cular, or n little oval transversely, much compressed or nearly flat, excepting 

 in the central and umbonal regions, which are moderately convex; beak 

 rather oblique, projecting little above the hinge, and bul slightly incurved; 

 surface (of an internal cast) ornamented with regular, rather prominent, sub- 

 angular, concentric undulations, separated by wider rounded depressions. 



Height of left valve, about 7 inches; antero-posterior diameter, 5.10 

 inches; convexity, 4.50 inches. Right valve, height, about 5.70 inches; 

 antero-posterior diameter, 5.10 inches ; convexity, about 1.60 inches. 



The description of this remarkable shell first published was made out 

 from a portion of a small left valve, giving a rather indefinite idea of the char- 

 acters of the species. Since that time, I have had an opportunity to examine 

 some fine large specimens, brought from the same locality by Lieutenant 

 Mullan's party, and it is mainly from these that the above description is made 

 out. It is true, none of these specimens show the two valves connected, but 

 there are among them right and left valves, bearing so exactly the relations 

 to each other that the right and left valves of the most nearly analogous 

 European species do, that I have scarcely a doubt in regard to their being 

 the opposite valves of one species. 



I had at first remarked that this species is related to /. involutus of 

 Sowerby, but I was not at that time aware how very closely if is allied to 

 Sowerby's species.' After a careful comparison of the additional specimens 

 alluded to, with figures and descriptions of I. involutus, they ai*e found to 

 agree in so many respects that I would not be surprised if a comparison of 

 specimens from these two distant localities should prove these shells to be 

 specifically identical. The only differences that have thus far been discov- 

 ered between them, are the following: In the first place, the antero-posterior 

 diameter of I involutus is less in proportion to the height of its left valve 

 than in our Nebraska shell. Again, the aperture of its left valve is more 

 nearly circular, being slightly higher than wide, while in the shell under 

 consideration, it is somewhat oval transversely, being slightly wider than 

 high. A more important reason, however^ for regarding these shells as 

 probably belonging to different species, is the fact that I. involutus occurs in 

 France and England in the Upper or White Chalk, while our Nebraska 

 shell comes from a formation we have reason to regard as equivalent to the 

 Lower or Gray Chalk. 



The most nearly allied American species of which I have any knowl- 



