INVEETEBBATE PALAEONTOLOGY. 87 



the posterior muscular impression. Partly decomposed specimens sometimes 

 show the outer lamina to have an obscurely subfibrous structure. Generally 

 the margins of the valves appear exactly equal, probably owing to the wearing- 

 away of the sharp, slightly more prominent ventral margin of one valve. 

 Sometimes, however, the ventral edge of the left valve is seen to project a 

 little beyond that of the right, which is received in an obscure linear groove, 

 just within the edge of the left valve. 



On first examining this species, we were struck with its remarkably close 

 similarity to C. fibrosa of Sowerby, as figured by d'Orbigny in the Palaeontol- 

 ogy of France; but, owing to the fact that the pallial margins of the valves in 

 all the specimens we had seen, appeared to be exactly equal, while that of the 

 right valve in Sowerby's species projects a little beyond the other, led us to 

 regard them as distinct species. Subsequently, in examining some additional 

 specimens received from the same localities, we discovered, in some of the 

 casts, unmistakable evidences of a very slight inequality of ventral margins of 

 the valves; and, as there thus appeared to be no well-defined difference 

 between our shell and Sowerby's species, they were regarded as probably 

 identical, in making out our catalogue, published in the Proceedings of the 

 Acad. Nat. Sci. Philad., Oct. 1860. As C. fibrosa, however, is Gault and 

 Greensand species, while our shell has only been found in beds equivalent to 

 the true Chalk, I am still left in doubt in regard to the identity of these shells, 

 and here retain provisionally our name Shumardi, until the question can be 

 settled by comparison of specimens. Another reason for doubting the identity 

 of these shells, is the fact that it is the ventral margin of the left valve in the 

 Nebraska specimens (so far as I have been able to determine) that projects 

 beyond the other; while, according to d'Orbigny, it is that of the right in 

 Sowerby's species. Our shell seems also to have a narrower cardinal area. 

 It is likewise very closely allied to C. Tippana of Conrad, from the Cretaceous 

 rocks of Mississippi, but differs in being less distinctly inequivalve, and in 

 not having the margin of the larger valve thickened. 



Locality and position. — Moreau River, Fox Hills, Long Lake, &c; Fox 

 Hills group, or formation No. 5 of the Cretaceous series of the Upper Mis- 

 souri. 



