572 UNITED STATES GEOLOGICAL SURVEY OF THE TERRITORIES 



ovate, obtusely angular above, and very narrowly rounded or subangular at 

 tbe base ; outer lip straight in outline ; inner lip free from the columella 

 below, so as to leave a small, deep, umbilical perforation, very slightly pro- 

 duced and sinuous at its connection with the outer lip at the base of tbe 

 aperture 



Length, 0.18 inch; breadth, 0.10 inch; length of aperture, 0.06 inch; 

 breadth of same, 0.04 inch ; apical angle slightly convex, divergence 40°. 



On some of the specimens, by the aid of a good lens, in a favorable 

 light, faint traces of exceedingly minute revolving striae are seen. This char- 

 acter, however, is so faintly marked that it might be readily overlooked, and, 

 indeed, can rarely be seen even on well-preserved specimens. 



Locality and position. — Yellowstone River, thirty miles above its mouth; 

 from the Fort Union group of the Brackish- and Fresh-water Lignite series ; 

 probably Eocene. 



Hjdrobia Warrenana, M. & H. 



Plate 43, figs. 11, a, b, c. 



i 



Mtlania Warrenana, Meek and Haydeu (1857), Proceed. Acad. Nat. Sci. Pbilad., IX, 137. 

 Hclanial Warrenana, Meek and Haydeu (1860), ib., XII, 130. 



Shell small, conical ; axis not perforated ; spire moderately elevated, 

 pointed at the apex ; volutions about seven, nearly flat or but slightly convex, 

 closely coiled and increasing gradually in size ; body-whorl not large, rather 

 prominent or subangular around the middle ; suture moderately well defined, 

 though not deep; surface apparently smooth, but showing under a good 

 magnifier very obscure lines of growth, and exceedingly faint traces of 

 minute revolving stria? ; aperture subovate, a little oblique, narrowly rounded 

 or obscurely subangular, and faintly sinuous below ; lip nearly straight above, 

 and scarcely prominent in outline below the middle. 



Length, 0.29 inch; breadth, 0.14 inch; apical angle slightly convex, 

 divergence 32°. 



This little shell may not be a true Hydrobia, but I have concluded to 

 place it provisionally in this genus until other specimens can be examined. 

 It is a neat symmetrical shell, resembling the last more nearly than any 

 other form with which I am acquainted from these rocks. It is considerably 

 larger, however, and has about one and a half to two more whorls, with a 

 proportionally longer spire, and differs in not having its axis perforated. 

 Among foreign Tertiary species, it seems to be most nearly represented by a 



