538 UNITED STATES GEOLOGICAL SURVEY OE THE TERRITORIES. 



P ! a 11 o r b i s ronvolutiis, v a r . 



Plate 42, figs. 11, a-e. 



Fliis form differs from the typical specimens of P. convolutus chiefly in 

 having its whorls less compressed, especially on t lie upper side. It is also 

 more concave on both sides, and its aperture appears to be proportion- 

 ally higher and less oblique. It may possibly be specifically distinct ; but 

 with the specimens yet at hand for comparison, I am inclined to believe it 

 only a variety of the last. 



Locality and position. — Same as last. 



Planorbis (fiathyomplialus) planoconvex us, M. & H. 



Plato 44, figs. 9, a, b, c. 



Planorbis fragilis, Meek aud Hayden (1857), Proceed. Acad. Nat. Sci. Philad., IX, 136. (Not F.fragilis, 



Danker, 1843.) 

 Planorbis planoconvexus, Meek and Hayden (I860), Proceed. Acad. Nat. Sci. Philad., XII, 185. 



Shell rather large, thin, plano-convex, the upper side being nearly or 

 quite flat, and the under side convex; umbilicus nearly one-third wider than 

 the outer whorl, deep, and showing a part of each inner whorl; sutures 

 linear, most distinct on the upper side; volutions about five or six, flat above, 

 angular around the upper outer margin, and convex and sloping inward below 

 from the periphery to the margin of the umbilicus, which is also very prom- 

 inent or obtusely angular; inner side forming the walls of the umbilicus 

 distinctly concave, each whorl rather more than three-fourths hidden by the 

 succeeding turn on the underside; aperture subtrigonal, or unequally four- 

 sided, the upper side being nearly straight and horizontal, the lower outer 

 side (which is oblique and about of the same length) convex, and the two 

 shorter inner sides next the umbilicus and the succeeding whorl, concave; 

 surface marked by moderately distinct lines of growth. 



Greatest breadth, 1.26 inches; height, 0.38 inch. 



This shell is remarkably distinct from all the other known species of 

 this country, either living or fossil. Among foreign fossil species, its nearest 

 representative known to me is P. euomphalus of Sowerby, from the English 

 Eocene deposits. It may be readily distinguished, however, by its less com- 

 pressed form, and smaller as well as deeper umbilicus. Its whorls are also 

 more angular around the margins of the umbilicus, and more concealed on 

 the under side ; while their inner sides forming the walls of the umbilicus 

 are more concave. 



