INVERTEBRATE PALEONTOLOGY. 323 



one of the angles being situated at the termination of each one of the rapidly- 

 diverging carinae of the body- volution; aperture rather oblique, narrow-oval, 

 terminating above in tin 1 posterior canal (which extends far up the spire, 

 becoming free at the end),* narrowed into the small canal below; surface 

 marker! by obscure lines of growth, and faint indications of nearly obsolete, 

 very flexuous, vertical costal, which are crossed by very fine, closely-arranged, 

 revolving striae. 



Length, 0.53 inch ; breadth of body-volution, exclusive of the extended 

 lip, 0.21 inch; slopes of spire nearly or quite straight, with a divergence 

 from the apex of about 34°. 



The costee mentioned above are only seen on the volutions of the spire, 

 and are very obscure, or possibly sometimes entirely obsolete. Of the two 

 smooth keels around the body-volution, the upper one is a little larger than 

 the other. They pass around nearly parallel to each other, until they reach 

 the commencement of the expansion of the lip, where they diverge and cross 

 the lip, one extending to each of the marginal angles of the same. The edge 

 of the lip is very broadly and slightly sinuous, horizontally across below, 

 from the lower angle to the beak, and nearly vertically between the two 

 angles, as well as obliquely inward and upward from the upper, and slightly 

 more prominent angle, to the posterior canal ascending the spire. 



Mr. Gabb, in his revision of the family Aporrhaidce, makes the inquiry 

 whether this species may not rather belong to his subgenus Gon'wcheila than 

 to Aporrhais proper, and seems to doubt the existence of the typical forms of 

 Aporrhais in the Cretaceous. A glance at our wood-cut figure, however, 

 which he had not seen, will show that our shell certainly has not the charac- 

 ers of his subgenus; its lip being decidedly and almost equally biangular.its 

 beak not incurved, and, as our other specimens show, its posterior canal as in 

 Aporrhais proper. I can therefore see no other differences, than such as may, 

 I think, be fairly considered specific, between it and typical forms of the 

 genus. 



Locality and position. — Yellowstone River, 150 miles from its mouth, in 

 beds containing a blending of the fossils of the Fort Pierre and Fox Hills 

 groups of the Upper Missouri Cretaceous series. 



"The specimens from which our figures ou plate 19 were drawn, have the posterior canal broken 

 away. Others were subsequently found showing it to extend up the spire as in typical forms of the 



genus. 



