31(7 UNITED STATES GEOLOGICAL SURVEY OF THE TERRITORIES. 



on the inner side, widest and rounded below, subangular above ; outer lip 

 sharp ; inner lip slightly thickened, and a little reflexed on the body-volution 

 above the small umbilical perforation. 



Length about 1 inch; breadth, 0.73 inch ; length of aperture, 0.57 inch; 

 breadth of aperture, 0.34 inch ; apical angle, a little convex, divergence, 70°. 



This shell, in some of its variations, is so very nearly like the last that 

 I have some doubts whether it should not rather be regarded as a variety of 

 that species. Still, it differs so much in the elevation of the spire that it 

 would scarcely seem possible that they can be mere varieties of the same 

 species. It also has one volution more than the last, and its second whorl is 

 proportionally larger and more convex. This latter character is sometimes so 

 marked as to give it a constricted appearance around the suture, between the 

 first and second volutions. Its body-whorl is likewise often more oblique 

 and less convex above, near the aperture, than in the last. 



The revolving lines seen on both of these forms are very obscure, and 

 indistinctly visible without the aid of a lens. Where the surface is a little 

 worn or weathered, they are often obsolete. 



Locality and position. — Moreau River, Dakota ; from the Fox Hills group 

 of the Upper Missouri Cretaceous series 



Lnnatia s h b c r a s s a , M. & H. 



Plate 39, figs. 3, a, b, c. 



Katica subcrassa, Meek and Haydeu (1856), Proceed. Acad. Nat. Sci. Philad., VIII, 67. 

 Natica (Lunatia) subcrassa, Meek and Hayden (1860), ib., XII, 422. 

 Lunatia subcrassa, Gabb (1861), Synops. Moll. C'ret. Form., 58. 



Shell rather thick, obliquely compressed-subglobose or subovate ; spire 

 short; volutions three and a half to four, last one comparatively large, sloping 

 obliquely outward and downward almost immediately from the suture above, 

 and rounded below the middle ; suture distinct, but not deep; aperture large 

 subovate, being angular above, rounded below, forming a semi-ovate curve on 

 the outer side, and nearly straight on the inner; outer lip very oblique, being 

 much produced forward above, and extending obliquely backward, with a 

 broad and slightly sinuous outline near the middle ; inner lip nearly straight, 

 moderately thick, and a little reflexed upon the body-volution above the small 

 umbilical perforation; surface showing (on slightly-weathered specimens) 

 distinct, very oblique lines of growth. 



Length, 1.08 inches; breadth, 0.90 inch ; length of aperture, 0.85 inch, 

 breadth of same, 0.57 inch. 



