396 GEOLOGY OF THE BLACK HILLS. 



size. Surface of the cast marked by oblique curving lines, or lines of pus- 

 tules, which incline slightty forward as they approach the basal margin. 



This species may be distinguished from any other form described by 

 the great postero-basal extension of the valves, coupled with their mod- 

 erate gibbosity. In its younger stages of growth, when within two inches in 

 length, it has resembled the forms referred to /. vanuxenii by Mr. Meek, but 

 that it would have been much longer in proportion to its height. The finer 

 and more equal undulations and smaller beaks will readily distinguish it 

 from I. sagensis Owen, or any of the forms referred to that group. The 

 cartilage area appears to have been very narrow and linear ; but the 

 most noticeable feature, and one that will easily distinguish it, we think, 

 when the valves are found in contact, is the greater convexity of the right 

 valve, and apparent greater prominency of the beak of this valve, whereas 

 the opposite is usually the case with all species of this genus. The beaks 

 of the right valves are all broken, and we judge only of their size by the 

 parts remaining. The specimen figured by Mr. Meek, loc. cit., differs from 

 our figured specimen in being narrower posteriorly and in the slight con- 

 striction or sinuosity of the basal margin, but we do not deem these features 

 of specific importance. The type of the species was from a lower horizon 

 than most of those in this collection. 



Formation and locality. — In Cretaceous rocks of the Fort Pierre" Group, 

 on Old Woman Fork ; one specimen is marked as coming from the east fork 

 of Beaver Creek, three miles west of Camp Jenney, Black Hills, at a prob- 

 ably somewhat lower horizon. 



INOCEBAMUS VANUXEMI. 



Plate 7, figs. 8, 9 ; Plate 8, figs. 4, 5. 



Inoceramus vanuxemi M. & H., May, 1860, Froc. Acad. Nat. Sci. Phil., p. 180. 

 Inoceramus mortoni M. & H., Oct. 1860, Proc. Acad. Nat. Sci. Phil., p. 428. 

 Inoceramus proximus M., Pal. U. S. Geol. Surv. Terr., p. 53, PI. 12, fig. 7; and var. 



subcircularis M., ib., p. 55, PI. 12, fig. 2. 

 Compare Inoceramus proximus Tourney, Proc. Acad. Nat. Sci. Phil. 1854, p. 428. 

 Compare Inoceramus confertim-annnlatus Boemer, Kreid. von Texas, PI. 7, fig. 4. 



Shell of moderate or large size, with depressed convex to subdiscoid 

 valves, subcircular to subovate in outline, with short, compressed, often 



