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 generall}' increasing in size 

 and strength with the increased orowth 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. 



INOCERAMUS FEAGILIS. 



Plato 9, fig. 10. 



Inoceramtis fragiUs H. & M., Mem. Aui. Acad. Arts and Sci., new ser., voL viii, p. 388, 



PL 2, f. C. 

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



PL 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 be}'ond 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 extremitj'^, where it is somewhat sharpl}^ rounded. Body of 

 the valve irregularly convex, most prominent on the iimbonal 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 con.siderable 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 liaving the surface linings developed into irregular concentric folds of 

 greater or less strength. The I'fght valve is, so far as can be ascertained 



