TO JUNCTION OF GRAND AND GREEN RIVERS. 123 



Jersey and in the Upper Chalk of Europe ; as well as on the North Platte, in the Fox 

 Hills Group of the Upper Missouri Cretaceous section. 



Although it has been found convenient, throughout considerable areas on the 

 Upper Missouri, to subdivide the equivalents of the Middle and Upper Divisions of the 

 New Mexican section each into two groups, this is not everywhere the case, and Dr. 

 Newberry did not find any well-marked physical breaks in either of these divisions in 

 New Mexico. 



It is worthy of note that, as on the Upper Missouri, the strongest paleeontological 

 break is just where Dr. Newberry draws the line between his Middle and Upper 

 Divisions. Up to this time, not a single species is known to cross this line, either on 

 the Upper Missouri, in Kansas, Nebraska, Texas, or in New Mexico. 



A careful study of extensive collections of these fossils, brought by Dr. Ilayden 

 from these rocks on the Upper Missouri, has satisfied me that the beds above this 

 horizon represent the Upper or White Chalk of Europe ( "Etage Senonien," 

 d'Orbigny); while those below (from which we have yet obtained fossils) represent 

 the Lower or Gray Chalk, and probably the Upper Greensand of Europe (= the 

 Turonien and Cenomanien, d'Orbigny).* The evidence, so far as it goes, to be 

 derived from the New Mexican collections under consideration, warrants the same con- 

 clusion in regard to the rocks from which they were obtained. 



LAMELLIBRANCHIATA. 

 Genus OSTREA, Linn. 

 OSTREA LUGUBRIS, Conrad. 



Plato I, figs. 1 a, b, c, d. 

 (Mrt'ti liiijubiiK, Conrad (1^57), U. S. anil Hex. Bound. Report, vol. 1, p. 15(i, phito 10, tig. 5 a, b. 



Shell small, thin, oval, or subcircular. Lower valve attached by most of its under 

 surface; while its lateral and ventral margins rise abruptly, usually for a short distance, 

 above the base of attachment, becoming at the same time ornamented by small regular 

 plications, which are sometimes rendered slightly rugose by marks of growth; beak 

 small, obtusely angular, usually turned a little to the left, and attached by its whole 

 under side; area short, triangular; muscular scar transversely ovate-reniform, and 

 placed near the left or anterior side. Upper valve flat, or sometimes a little convex 

 near the umbo, and slightly concave in the middle; marked by small regular plications, 

 which rarely extend to the umbo, and are often confined to the free margins. 



Length of a large specimen, 0.95 inch; breadth of same, 0.65 inch; depth vary- 

 ing from 0.12 to 0.28 inch. 



This seems to be a common and characteristic species in the lower portions of the 

 M iddle Cretaceous of New Mexico. The specimens usually have a proportionally larger 

 scar of attachment than that figured by Mr. Conrad. Where they are attached by the 

 whole under side of the lower valve, the plicated margins rise little above the base of 

 attachment; but where the scar of adhesion is small, the free plicated margins are more 

 produced; though it seems never to be a very deep shell. It was not gregarious to the 



* Proceedings of the Academy of Natural Sciences of Philadelphia, January, l.-'IU. 



