390 GEOLOGY OF THE BLACK HILLS. 



Surface of the casts marked by strong, irregular, concentric undula- 

 tions parallel to the margin of the shell and generally increasing in size 

 and strength with the increased growth of the shell. 



Formation and locality. — In ferruginous sandstone of Cretaceous age, 

 on the east fork of Beaver Creek, Black Hills. Probably of the Fort 

 Benton or Niobrara Group. 



INOCEBAMUS FKAGILIS. 



Plate 9, fig. 10. 



Inoceramus fragilis H. & M., Mem. Am. Acad. Arts and Sci., new ser., vol. viii, p. 388, 



PI. 2, f. G. 

 Inoceramus fragilis (H. & M.) Meek, Pal. TT. S. Geol. Surv. Terr., p. 42, figs. 1 and 2, 



PI. 5, fig. 5. 



Shell rather below the medium size, obliquely rhomboid-ovate in out- 

 line, with moderately convex valves and small, somewhat pointed and 

 nearly terminal beaks, which scarcely project beyond the cardinal line; 

 hinge line straight, shorter, and sometimes not more than two-thirds as long 

 as the shell below ; anterior border straight for some distance below the 

 beaks, and at right angles with the cardinal line ; posterior end obliquely 

 rounded and prolonged in the direction of the postero-basal angle ; basal 

 line rounding at its junction with the anterior border and below, sub- 

 parallel with or slightly diverging from the line of the hinge to near the 

 postero-basal extremity, where it is somewhat sharply rounded. Body of 

 the valve irregularly convex, most prominent on the umbonal ridge, which 

 is often subangular and the surface somewhat flattened on each side, espe- 

 cially along the postero-cardinal slope ; anterior margin nearly vertical. 



Surface marked by fine, even, concentric lines and sometimes by con- 

 centric undulations, the latter, however, are not always fully developed and 

 are frequently entirely obsolete. 



There is considerable variation in the form of the outline of this shell 

 among the different specimens, many of them being less oblique and 

 rounded on the basal margin, the posterior extremity being less prolonged. 

 They also differ greatly in the strength of the surface markings, some of 

 them having the surface linings developed into irregular concentric folds of 

 greater or less strength. The right valve is, so far as can be ascertained 



