PROCEEDINGS OF UNITED STATES NATIONAL MUSEUM. 159. 
regular curve; beaks small, submedially located, distinct but incon- 
Spicuous; umbonal ridge slightly developed, curved, passing near the 
dorsal and posterior borders. Surface marked by the usual concentric 
lines of growth. 
Length, 20 millimeters; height, from base to beaks, 18 millimeters. 
(Museum No. 8362.) 
This species is readily recognizable by its comparatively narrow pos- 
terior side, its deeply convex basal border, and slight convexity of the 
valves. 
Position and locality Cretaceous strata, Monument Creek, Colorado, 
_ where it was obtained by Dr. A. C. Peale. 
GASTEROPODA. 
Genus PLANORBIS Guettard. 
PLANORBIS A2QUALIS (sp. NOv.). 
Shell rather small, coiled nearly in a plane, apparently sinistral; 
whorls apparently 4 or 5, in close contact but only slightly involute, 
broadly convex upon the periphery, but their sides more narrowly con- © 
vex, their transverse diameter greater than that which corresponds with 
the plane of the coil. 
Surface marked by a considerable number of revolving raised lines or 
slight angulations, which are crossed by the usual lines of growth. 
Diameter of the full coil of the largest example discovered, 6 milli- 
meters. (Museum No. 8909.) 
This is apparently the only species of typical biumbilicate Planorbis 
that has yet been discovered among the fossil fresh-water faunze of the 
Western region, and it therefore needs no detailed comparison. 
Position and locality—Green River Group, Eocene, Henry’s Fork of 
Green River, Southern Wyoming. i 
Subgenus GYRAULUS Agassiz. 
PLANORBIS (GYRAULUS) MILITARIS (sp. nov.). 
Shell very small, dextral, depressed-convex above, umbilicate below; 
volutions two and a half to three and a half, convex on all sides except 
the inner, which is very narrowly flattened against each preceding coil; 
suture deeply impressed both above and below; surface marked by com- 
paratively coarse lines of growth. 
Diameter of the full coil of the larger examples in the collection, 4 
millimeters. (Museum No. 8594.) 
This form was noticed but not named by me in vol. iv, U. S. Expl. 
& Sur. West of the 100th Merid., p. 210. At that time I was not sat- 
isfied as to the mature condition of these shells, but by careful examina- 
