420 GEOLOGY OF THE BLACK HILLS. 



tilage pit. The shell, in its external form, has all the features of Thracia, 

 being slightly inequivalve and twisted when viewed in profile, and also 

 in the character of the surface markings. The muscular impressions and 

 pallial line cannot be distinguished through the remaining shell and 

 we are therefore in doubt concerning these features. It bears consid- 

 erable resemblance to Thracia gracilis M. & H. (see Paleontology of the 

 United States Geological Survey of the Territories), 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, Black Hills, Dakota. 



Genus NE^RA Gray. 



NE^lEA MOREAUENSIS 



Plate 11, fig. 31. 



Corbula moreauensis M. & H., Proc. Acad. Nat. Sci. Phil., vol. 8, p. 83. 

 Necera moreauensis M. & H, ibid., vol. 12, p. 185. 



Necera moreauensis Meek, Pal. IT. S. Geol. Surv. Terr., p. 239, PI. 17, fig. 11. 

 Compare Necera ventricosa M. & H, ibid., p. 238, PI. 30, fig. 3. 



Shell minute, semi-globular, as seen in separated valves, with a small, 

 narrow, projecting posterior end, or if the valves are united would be 

 globular, with the extended posterior surface marked with concentric striae 

 or undulations, which, in proportion to the size of the shell, are quite strong. 

 The posterior extension of the valves is not more than one-fourth, or per- 

 haps one-fifth the height of the anterior part, and the surface depressed to 

 a corresponding degree. 



We are at a loss to which of Messrs. Meek and Hayden's species to 

 refer our shell, as they are so nearly alike, except in the character of the 

 surface, and occur in the same formation. More especially is this trouble 

 evident from the fact that the greater portion of the shell of our particularly 

 minute specimen has been removed, only a little remaining on the anterior 

 margin. 



Formation and locality. — In limestone of the Cretaceous, at the forks of 

 the Cheyenne River, Black Hills. 



