CRETACEOUS FOSSILS. 411 



This species differs from L. occidentalis, with which it is associated, in 

 being more nearly circular and more nearly equilateral, that one being 

 much longer behind than before, while this one is nearly equal. It also 

 differs in being wider posteriorly with a wider postero-cardinal slope, and 

 also in having the beaks a little more inclined forward We have noticed 

 specimens in several collections labeled L oceidentalis Morton, both separ- 

 ately and mingled with that species, but on comparing it with Dr. Morton's 

 figures we find them to be of the more elongate form, and this one is certain- 

 ly very distinct from those having that character before us. We noticed 

 also that Mr. Meek, in the Paleontology of the United States Geological 

 Survey of the Territories, just published, has placed it as a variety of L. ocei- 

 dentalis, expressing doubt, however, of the correctness of this view. They 

 are certainly much more distinct than many of the recent forms of the genus 

 considered as good species. Our specimens, which we have referred to the 

 L occidentalism are, however, proportionally longer, posteriorly, than those 

 he figures, and also much more like Dr. Morton's figures. 



Formation and locality. — In beds of the Fort Pierre Group, near the 

 top of the gray shales on the Cheyenne River near Rapid Creek, and at 

 the same horizon at other localities near by. A few specimens undistin- 

 guishable from the above are marked "11 M. S. of Belle Fourche," Black 

 Hills, Dakota. 



LUCINA (DIPLODONTA ?) SUBUNDATA. 



Plate 11, fig. 17, 18. 



Liicina subundata H. & M., Mem. Am. Acad. Arts and Sci., vol. viii, new ser., PI. 1, 



fig. 6. 

 Lucina subundata (H. & M.) Meek, Pal. U. S. Geol. Sur. Terr., p. 133, PI. 17, fig. 2. 



Shell small, orbicular in outline, with depressed convex valves; beaks 

 small, rounded, distinctly projecting above the general contour of the shell 

 and inclined forward; posterior side of the shell very slightly truncate, the 

 cardinal slope narrow and distinctly arched in its course between the beaks 

 and the posterior margin ; basal line rounded, more sharply so at a point 

 opposite the beaks; anterior to the beaks the cardinal surface of the shell' 



