434 GEOLOGY OF THE BLACK HILLS. 



Mr. Meek appears to think there may be some doubt of the true specific 

 relations of this form; that it niay possibly be only a young, or a variety of 

 A. siibovaius. The resemblance is indeed quite striking, differing in our 

 specimens only a little in the degree of elevation, slightly in the relative 

 position of the apex, and somewhat in its greater length. 



Formation and locality. — In limestones of the Fort Pierre* Group, on 

 Old Woman Fork of the Cheyenne River, Black Hills. 



AOTSOMYON SUBOVATUS. 



Plate 12, fig. 19. 



Helcion subovatus M. & H., Proc. Acad. Nat. Sci. Phil., vol. 8, p. 68. 

 Anisomyon subovatus M. & H., Am. Jour. Sci. and Arts 2d ser., vol. 28, p. 35. 

 Anisomyon subovatus (M. & M.) Meek, Pal. U. S. Geol. Surv. Terr., p. 291, PI. 18, figs. 

 5 d, and 6. 



Shell of moderate size, oval, slightly ovate or broadly elliptical in out- 

 line, the length and width as seven to five, or nearly so ; ends somewhat 

 regularly rounded ; sides nearly straight in the middle or only very slightly 

 convex ; height of the shell equal to a little more than half the width ; 

 apex small, recurved even on the internal cast, situated a little less than 

 one-third of the entire length from the anterior end ; anterior slope of the 

 shell forming nearly a straight line from the apex to the front margin ; 

 posterior slope quite convex ; sides slightly convex. Surface of the shell 

 apparently marked by moderately strong concentric undulations, as indi- 

 cated on the internal casts. 



Muscular imprints quite distinct, the anterior ends large, situated 

 directly on a line with the apex of the shell, the left side rapidly narrowing 

 posteriorly to near the middle of the posterior slope, where it is perceptibly 

 widened again on the right side, the anterior portion is oval and a little 

 oblique, the interruption is quite slight, and the posterior continuation 

 linear, with a second faint interruption farther back Anterior line con- 

 necting the opposite sides narrow, crossing the apex of the shell. There is 

 some variation among the individuals of this species, as is indicated by Mr. 

 Meek's figures, given in the Paleontology of the United States Geological 

 Survey of the Territories, and of course the description given above will 



