602 UNITED STATES GEOLOGICAL SURVEY OF TIIE TERRITORIES. 



tinctly angular whorls. The angle is also not so nearly central, and the 

 whorb are much less convex on the upper and lower sides, while its umbili- 

 cus is proportionally much wider. 



It bears considerable resemblance to P. Ungeri of Reuss and von Meyer 

 (Paheontographiea, II, 37, taf. 4, fig. 10), from the Miocene fresh-water 

 limestone in the northern part of Bohemia, but is not so distinctly angular 

 around the periphery, and its angle is not so near the middle of the outer 

 side. It also has usually about one whorl less, and more obscure surface- 

 markings. 



In the Eocene of the Old World, there arc also some very closely-allied 

 forms, among which may be mentioned P. lens, Bong., P. tropis, Edwards, 

 and P. Sowerbyi of Bronn. From the first, it differs in being a smaller 

 shell, in having about one whorl less, and a proportionally larger umbilicus. 

 It also differs in being a little more concave on the under than the upper 

 side, instead of the reverse. From P. Sowerbyi, which it very nearly resem- 

 bles, it appears to differ in being less sharply carinate, while its whorls are 

 not so flat on the under side, and increase less rapidly in size. It seems to 

 be even more closely related to P. tropis, but differs from Edwards' figures 

 of that species given in his Monograph of the Eocene Mollusca of England 

 (published by the Pateontographical Society), in the greater convexity of its 

 whorls on the under side, and in having a less distinctly-defined angle 

 around the periphery. It is desirable, however, that this Dakota shell 

 should be compared with authentic specimens of the above foreign forms, in 

 order to settle beyond doubt the question in regard to its relations to the 

 same ; though, with the present means of comparison, as well as from its 

 geogiaphical position, I can but regard it as distinct from them all. 



Our shell has also some near representatives among recent species, 

 being very similar in size and form to P. vermiculus of Gould from Oregon. 

 Still it differs in being slightly more compressed and subangular instead of 

 rounded on the periphery, while its umbilicus is a little wider and shallower 

 than in the Oregon species. 



Locality and position. — Same as last. 



