PROCEEDINGS OF UNITED STATES NATIONAL MUSEUM. 159 



regular ciu'vej beaks small, submedially located, distinct but iucou- 

 spicuous; umbonal ridge slightly developed, curved, passing near the 

 dorsal and posterior borders. Surface marked by the usual concentric 

 lines of growth. 



Length, 20 iniHimeters; height, from base to beaks, 18 millimeters. 

 (Museura 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. 



Fosition and locality. — Cretaceous strata, Monument Creek, Colorado, 

 where it was obtained by Dr. A. C. Peale. 



GASTEROPODA. 



Genus PLANORBIS Guettard. 

 Planorbis ^qualis (sp. nov.). 



Shell rather small, coiled nearly in a i)lane, apparently sinistral; 

 whorls apparently 4 or 5, in close contact but only slightly involute, 

 broadly convex upon the j)erii)hery, 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, G milli- 

 meters. (Museum No. 8909.) 



This is apparently the only species of typical biumbilicate Planorhis 

 that has yet been discovered among the fossil fresh-water fauniTB of the 

 Western region, and it therefore needs no detailed comi)arison. 



Fosition and locality. — Green Kiver Group, Eocene, Henry's Fork of 

 Green Eiver, Southern Wyoming. 



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 exce])t 

 the inner, which is very narrowlj^ flattened against each preceding coil; 

 suture deeply impressed both above and below ; surface marked bj' com- 

 paratively coarse lines of growth. 



Diameter of the full coil of the larger examjiles 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- 



