444 PROCEEDINGS OF THE ACADEMY OF 



so gentle a curve as to appear nearly parallel ; beaks rather obtuse, that of the 

 ventral valve being a little more prominent than the other ; valves nearly 

 equally convex, their greatest convexity being along the middle ; surface 

 polished, and only marked by fine lines of growth, which are obsolete on 

 the more convex part of the valves, but become rather distinct on each side. 



Length, 36 inch ; breadth, 0-10 inch ; convexity of the two valves about 

 0-10 inch. 



This species differs from L. subspatulata , Hall and Meek, (Mem. Am. Acad. 

 Boston, vol. 5, p. 380, pi. 1, fig. 2, a, b,) in being always much smaller, pro- 

 portionally narrower aud more convex, as well as in having its front very nar- 

 rowly rounded instead of subtruncate. It never attains one-fifth the size of 

 L. Rouliniana of D'Orbigny, (Pal. Fr. Ter. Cret. Brach.. p. 10, pi. 490, fig. 1,) 

 and has a much less pointed beak, as well as a more narrowly rounded front. 



It is often very difficult to distinguish the fossil species of this genus, 

 those coming from very widely different horizons being sometimes very nearly 

 alike, when we know from the vast extent of time that must have elapsed be- 

 tween the periods of their existence, that it is scarcely possible they can be 

 identical. 



Locality and position. Mouth Big Horn River, from near the horizon of the 

 base of the Fox Hills Beds of Nebraska Cretaceous series. 



GASTEROPODA. 



Genus NERIIELLA, Humphrey. 

 Neritella Nebrascensis. 



Shell small, obliquely ovdl ; volutions three to three and a half, increasing 

 rapidly in size, the last one composing more than nine-tenths of the entire 

 shell, all convex; suture well defined; aperture broad ovate, the inner side 

 being nearly straight; columella of moderate length, flattened, smooth, some- 

 what arched, and usually having a distinct opercular impression ; surface 

 smooth, or only having tine obscure marks of growth, and beautifully orna- 

 mented by alternate dark and light-colored, zigzag transverse bands. 



Height, 0-40 inch; greatest transverse diameter, measuring from the outer 

 side of the aperture, obliquely upwards to the most prominent part of the body 

 -whorl on the other side, 0-50 inch; height of aperture. 0-35 inch; breadth of 

 do., 0-25 inch. 



Locality and position. Head Wind River, from a bed apparently holding a 

 position at the base of the Cretaceous. 



Genus MELANIA, Lamarck. 



Melania (Potodoma) veterna. 



Shell conical-subovate ; spire moderately elevated, rather pointed at the im- 

 mediate apex; volutions six, very convex, rounded in the middle, and some- 

 times slightly compressed above; suture deep; aperture obliquely oval,_ nar- 

 rowly rounded below, and obtusely subangular above, much more prominent 

 on the outer than the inner side; inner lip a little thickened, somewhat arched, 

 and often slightly disconnected from the body whorl below, so as to leave a 

 small umbilical impression; outer lip thin, inversely sigmoid, or broadly sinu- 

 ous above the middle and at the base of the aperture; surface marked by 

 strono- flexuous strias of growth, crossed by more or less distinct thread-like 

 revolving lines, which are much more closely arranged on the lower half of the 

 body whorl than above. 



Length, 0-77 inch, breadth, 0-50 inch; apical angle very convex, diver- 

 gence about 47. 



[Dec. 



