INVERTEBRATE PALAEONTOLOGY. 537 



five and a half or more, increasing rather gradually in size, rounded on the 

 oilier side, concave within, and slightly more broadly rounded above than 

 below ; sutures well defined. 



Greatest diameter of largest specimen (which is not complete at the 

 aperture), 1.10 inches; height or convexity, 0.38 inch. 



All the specimens of this species in the collection are smooth silicious 

 easts of the interior, retaining scarcely any traces of the surface-markings. 

 On some of the larger individuals, a few obscure undulations of growth are 

 sometimes seen, passing around the whorls very oblicpiely backward from the 

 suture on the upper side, apparently parallel to the direction of the lines of 

 growth, so as to indicate that the lip extended considerably forward above. 

 The aperture, as near as can be determined from transverse sections of the 

 whorls, is generally a little oblique, and approaching an obovate form, the 

 lower side being narrower than the upper, and the inner side more or less 

 sinuous. 



This species bears considerable general resemblance to P. rotundatus, 

 Brard (Deshayes, An. sans Verteb. Env. Paris, SuppL II, pi. 47, figs. 1-5), 

 but presents several well-marked differences, being more concave on the 

 upper side, and having its whorls more rounded above, while they appear to 

 be always destitute of the revolving strife of that species, and want the angle 

 sometimes seen on the lower side of those of P. rotundatus. The form of its 

 aperture is also quite different, being more oblique, and higher than wide 

 instead of wider than high. 



It seems to be more nearly related to a Wyoming species described by 

 me many years since, under the name P. spectdbilis, from near Fort Bridger 

 (see plate 17 of the Palaeont. of King's Survey of the Fortieth Parallel, figs. 

 13, a-f, not yet published), and at one time I had concluded that the Wyo- 

 ming shell might belong to the same species. A more critical comparison, 

 however, leads me to think them distinct ; the P. spectabilis having constantly 

 a deeper umbilicus, and differing somewhat in the form of its aperture, and 

 some other details. 



Locality and position. — Little Horn and Powder Rivers, Montana; the 

 specimens were brought by traders, and seem to have been found loose; but 

 they probably either belong to the Judith River or Fort Union beds. They 

 agree most nearly in their state of preservation with the fossils from the 

 Judith River beds. 

 08 II 



