36 



Geol. Surv. Territ., which we have just received, have concluded it to 

 be a different species. It differs, first, in the greater size and more 

 robust habits, 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 somewhat. 



Formation and locality. In Cretaceous strata at Dead Man's Rapids, 

 Upper Missouri. Probably of No. 5. 



FAM. AXATINID J]. 



GENUS THRACIA, Leach. 

 THRACIA SUBGRACILIS, n. sp. 



Plate 11, figs. 29 and 30. 



Shell small, transversely ovate and slightly iuequivalve, valves con- 

 vex, veutricose in the middle, but becoming quite compressed and al- 

 most constricted behind ; anterior part large, forming nearly or quite 

 four-fifths 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 constricted and narrowed $ beaks small and com- 

 pressed, 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 con- 

 siderable 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 cartilage-pit. The shell, in its external form, has 

 all the features of Thracia, being slightly inequivalve and also slightly 

 twisted when viewed in profile, and also in the character of the surface 

 markings. The muscular impressions or pallial line cannot be distin- 

 guished through the remaining shell, and we are therefore in doubt con- 

 cerning these features. It bears considerable resemblance to Thraqia 

 gracilis M. & H. (Pal. U. S. Geol. Surv. Territ., p. 224, PI. 30, fig. 6), 

 but is proportionally shorter and somewhat more contracted in height 

 posteriorly. 



Formation and locality. In Cretaceous rocks, probably near the top 

 of No. 4; near French Creek, on the Cheyenne River, Dakota. 



