CRETACEOUa FOSSILS. 435 



not agree in its details with all specimens that may properly be referred to 

 the species. 



Formation and locality. — In limestones of the Fort Pierrd Group on Old 

 Woman Fork; associated with A. alveolus and JEndocostea typica. 



ANISOMYON PATELLIFORMIS. 



Plato 12, figs. 17, 18. 



Helcion patcU[formis M. «& H., Proc. Acad. Nat. Sci. Phil., vol. 8, p. (JS, 

 Ankomyon patdUjotmin M. & H., Am. Joui'. Sci. aud Arts, vol. 28, p. 35, PI. 1. 

 Anisomyon patelHformis (M. & U.) Meek, Pal. U. S. Geol. Surv. Terr., p. 290, PI. 18, 

 figs. 5 a, &, c, and /. 



Shell of medium size, slightly ovate or very broad-oval in outline, the 

 width equaling about four-fifths of the length. Surface of the shell ele- 

 vated, the extreme height about equaling one-half of the greatest width ; 

 apex obtuse and apparently very little incurved, situated nearly over the 

 anterior end or but little within a vertical line above the margin ; anterior 

 margin of the shell rounded ; posterior end a little more broadly rounded 

 and the sides forming regular elliptical curves. Anterior slope abrupt and 

 slightly concave ; posterior slope highly convex ; lateral slopes concave in 

 front of the middle and convex behind. Surface of the shell unknown ; 

 that of the cast marked by nearly obsolete undulations ; muscular impres- 

 sions very distinct, the anterior portions large and subcircular, posterior 

 parts narrower, except at the right posterior angle, where it again widens, 

 interi'uption distinct ; anterior connecting band linear, crossing the summit 

 of the shell at a distance posterior to the apex, equal to about one-fifth of 

 the length of the shell. 



Our specimens of this shell differ somewhat from those figured by Mr. 

 Meek, but not sufficientl}' so to be considered of much importance. The 

 apex is more obtuse and slightly inclined downwards, and is also situated 

 nearer the anterior margin, while the elevation of the shell and also its 

 relative convexity are greater. 



Formation and locality. — In limestone of the Fort Pierre Group, on Old 

 Woman Fork, Black Hills; associated with A. alveolus and A. subovatus. 



