600 UNITED STATES GEOLOGICAL SURVEY OF THE TERRITORIES. 



subcircular, oblique, a little oval, flattened or somewhat concave on the 

 inner side. 



Greatest breadth, 0.22 inch : height, 09 inch. 



The more rounded, ventricose whorls, and small, deep umbilicus of this 

 species will serve to distinguish it at a glance, from either of the follow- 

 ing two forms. It will also be found, on comparison, to have an entirely 

 differently-formed aperture, and to be destitute of the more or less distinctly- 

 defined angle characterizing those shells. It is likewise much less com- 

 pressed than either of them, and has a narrower, deeper umbilicus. 



In form, it bears considerable general resemblance to our Valvata sub- 

 umbilicata from the Lignite basin, but is a larger shell, and differs in having 

 the spire slightly concave, instead of rising a little above the body-wliorl ; 

 while the form of its aperture, as might be expected, is quite different. The 

 specific name' was given in honor of Professor Joseph Leidy, the distin- 

 guished comparative anatomist of Philadelphia. 



This shell is rather remarkable for its very narrow, deep umbilicus, 

 which seems to separate it from the typical section of the genus. 



Locality and position. — Head of White River, Dakota; in the upper 

 part of the White River group ; Upper Eocene or Lower Miocene. 



Subgenus MENETUS, H. and A. Adams ? (see page 536). 

 Plan or bis Nebrascensis, E. & S. 



Plate 42, tigs. 2, «,/>. 



Planorbis Nebrascemsis, Evans and Shnmard (1854), Proceed. Acad. Nat. Sei. Philad., VI, 161. 

 PlanorMs (Segmcntina ?) Nebrascenais, Meek and Haydeu (I860), ib., IX, 431. — Meek (1864), Smithsonian 

 Cbeck-List North American Tertiary Fossils. 13. 



Shell small, depressed-discoid, with upper and lower sides nearly equally 

 and moderately concave, the umbilical concavity being slightly narrower than 

 that above; periphery angular; volutions about three, convex both above 

 and below, all coiled almost exactly on the same plane, more or less angular 

 around the outer side, and deeply sinuous within for the reception of the 

 next inner turn — each covering a little less than half of the next within, on 

 tin; upper side, and slightly more below; aperture cordate-subsagittate ; sur- 

 face showing very obscure lines of growth. 



Greatest diameter about 0.19 inch ; height or convexity about 0.06 inch. 



Our specimens of (be form referred with some little doubt to the above 

 species are from the same locality and position as tliuse described by Doctors 



