CEETACEOUS FOSSILS. 419 



the shell, which has the appearance of a thick epidermis, and in some places 

 is excorticated. 



We were at first inclined to consider this as C. undidata M. & H., but 

 on strict comparison with Mr. Meek's description in the Paleontology of the 

 United States Geological Survey of the Territories, which we have just 

 received, have concluded it to be a different species. It differs in the 

 greater size and more robust habit, also in the comparatively stronger 

 undulations, and in having the radiating costse continuing to the anterior 

 end, if not to the posterior also. The outline of the shell also differs some- 

 what. 



Formation and locality. — In Cretaceous strata at Deadman's Rapids 

 Upper Missouri. Probably of No. 5. 



ANATINID^E 



Genus THRACIA Leach. 



THEACIA SUBGEACILIS. 

 Plate 11, figs. 29, 30. 

 Thracia subgracilis Whitf., Brelim. Sept. Fal. Black Hills, 1877, p. 36. 



Shell small, transversely ovate, and slightly inequivalve. Valves con- 

 vex, ventricose in the middle, but becoming quite compressed and almost 

 constricted behind ; anterior part large, forming nearly or quite four-fifth, 

 of the entire bulk of the shell, the extremity rounded; basal line full in front 

 of the beaks and constricted or sinuate behind ; posterior end narrow and 

 subtruncate ; anterior cardinal line long, nearly on a line with the beaks, 

 and very slightly convex ; posterior to the beaks the cardinal line is con- 

 stricted and narrowed ; beaks small and compressed, appearing as if directed 

 backward. 



The surface of the only specimen is imperfect, the shell being in great 

 part removed, but where preserved is marked by concentric lines of consid- 

 erable strength, approaching in character to undulations rather than striae. 

 The beaks are small and marked along their apices, and for a short distance 

 below, by a slight notch or slit from the removal of a narrow ridge which 

 has marked the interior of the shell, as in Anatina, and supporting the car- 



