INVERTEBRATE PALEONTOLOGY. 35 



about half-way between 1 he middle and the anterior end of the same; sur- 

 face ornamented by numerous small, thread-like, radiating lines, less at and 

 near the free margins, than the breadth of the spaces between, in some of 

 which latter a smaller line is often intercalated; very minute concentric striae 

 are also to be seen on well-preserved specimens by the akl of a magnifier. 

 Right valve with a broad comjiressed alation behind, that seems not to 

 extend into a defined wing; posterior margin truncated and nearly or quite 

 straight; anterior wing very small, and defined by the usual deep, sharply- 

 cut byssal sinus of the subgenus Oxytoma; surface appearing nearly smooth, 

 lint when examined under a good magnifier, showing obscure traces of small, 

 radiating costas, crossed by extremely fine, regular, crowded, concentric stria?; 

 beak more compressed than that of the other valve, and a little less promi- 

 nent. 



Greatest length of a medium-sized specimen, measuring from the end of 

 the anterior wing obliquely to the posterior basal margin, 0.45 inch ; length 

 of hinge, 0.32 inch ; convexity of the two valves, about 0.15 inch. 



In comparing our description with that given by Evans and Shumard, 

 it should be remembered that they only had the left valve, and knew nothing 

 of the right. Their description, however, agrees so well with the left valve 

 of the specimens under consideration as to leave little or no room for doubt- 

 ing its identity with the species described by those gentlemen. The oppo- 

 site valves of these shells differ so much in form and surface-markings that 

 they would hardly be supposed to belong to the same species if not found 

 united. 



This species will be readily distinguished from the last by its less oblique 

 form, more prominent posterior and less prominent anterior wings, and much 

 liner radiating markings. The beak of its left valve is also less elevated than 

 that of the last. Its right valve reminds one of a species figured by Mr. 

 Conrad, from the Cretaceous of New Jersey in 1854, under the name Aviculu 

 abrupta (Jour. Acad. Nat. Sci. Philad., 2d. ser., vol. II, pi. 4, figs. 5 and 6), 

 which is evidently an Oxytoma. As Mr. Conrad says nothing about surface- 

 markings on his species, however, it would seem to be distinct. 



Locality and position. — Fox Hills, Moreau River, South Fork of the 

 Cheyenne, Milk River above Fort Union, and on the Yellowstone. It ranges 

 from the upper part of the Fort Pierre group, through the Fox Hills group 

 of the Upper Missouri Cretaceous. I also identified it among Professor 



