INVERTEBRATE PALEONTOLOGY. (501 



Evans and Shumard, and agree very nearly with their description, excepting 

 in apparently never having more than about three volutions. They are plan- 

 orbicular in form, the spire being a little concave, and the umbilicus, which 

 is slightly narrower and deeper than the concavity on the upper side, very 

 nearly of the same breadth 'as the outer whorl. The volutions are nearly 

 equally convex above -,[\\d below, obtusely angular on the outer side near the 

 middle, and deeply grooved within for the reception of the inner whorls, 

 each of which is from one-third to one-half embraced by the succeeding 

 turn. The aperture is subcordate, not oblique, and slightly wider than high. 

 The marks of growth are obscure. 



This and the following species seem to have the external characters of 

 (he genus Segmenlina ; but it is impossible to determine whether or not they 

 possess the internal teeth of that genus. My present impression is that they 

 may belong to the subgenus Menetus, H. and A. Adams. 



Locality and position. — Pinots' Creek, upper part of White River group, 

 of the Tertiary series. 



Planorbis vetustus, M. & H. 



Plate 45, figs. 1, a, b, c. 



Planorbis vetustus, Meek and Hayden (1860), Proceed. Acad. Nat. Sci. Philad., IX, 175. 



Planorbis (Segmentina ') vetustus, Meek and Hayden (1860), ib., XII, 431. 



Planorbis vetustus, Meek (1864). Smithsonian Check-List N. Am. Tertiary Fossils, 13. 



Shell small, discoidal, much depressed; spire slightly concave ; umbili- 

 cus shallow, very little broader than the concavity on the upper side, and 

 rather more than one-third wider than the outer whorl, showing about half 

 of each inner turn ; volutions three and a half to four, compressed-convex 

 above and below, the upper side being a little more convex than the other, 

 and sloping slightly outward from near the inner margin ; all rather distinctly 

 angular around the outer side somewhat below the middle, and deeply 

 grooved within for the reception of each succeeding inner whorl; sutures 

 well defined, though not very deep ; aperture narrow-subcordate, approach- 

 ing an irregular hastate outline, very slightly oblique, having its longer axis 

 in the direction of the greatest breadth of the shell; surface apparently 

 nearly smooth, or only showing obscure marks of growth. 



Greatest breadth, 0.23 inch ; height, 0.05 inch. 



This shell might be confounded with the last on a hasty examination, 

 but may always be distinguished by its more depressed form and more dis- 

 70 ii 



