38 

 APORRHAIS (G-ONIOCHEILA) CASTORENSIS, n. sp. 



Plate 12, fig. 1. 



Shell small, with a moderately elevated spire, composed of about four 

 flattened or very slightly veutricose volutions, which are crossed by fine 

 flexuous, vertical folds, strongly directed forward in their course across 

 the whorl, and also marked by fine thread-like revolving lines ; suture 

 distinct; apical angle about 40, but slightly variable on different 

 individuals. Body volution proportionally large and very strongly 

 angular, or even carinate, along the middle, flattened or sMghtly concave 

 on the upper surface and rapidly contracted below to the short, pointed 

 rostral beak. A second rather indistinct carination marks the surface 

 a little below the first, but seldom or never extends to the margin of the 

 lip. Outer lip expanded, strongly carinate on the back and projecting 

 in the middle, to form a short, obtuse, slightly recurved digitation, and 

 posteriorly extending along the spire to the base of the second volution 

 above. 



This species somewhat resembles A. biangulata, M. & H. (Pal. U. S. 

 Geol. Surv. Territ., p. 322, pi. 19, fig. 6), but differs in the subdued 

 character of the lower carination and in the strongly uuiaugular form of 

 the body volution. Among a number of specimens none show the pos- 

 terior canal extending above the point described, nor any evidence of a 

 second digitation to the lip. The surface markings are quite superficial, 

 but few specimens showing them, appearing quite smooth from slight 

 exfoliation. 



We are extremely averse to describing new species of this group of 

 shells, as, from their extreme liability to variation with different degrees 

 of development, they are easily mistaken ; and we have feared that this 

 might prove, on the examination of a larger and better collection of 

 specimens, to be only a form of A. Uangulata, M. & H., above referred 

 to; but the single strong carination, and the fact that it comes from a 

 lower geological horizon and distant locality, have induced us to separ- 

 ate it under a new name. 



Formation and locality. In a ferruginous sandy limestone, on the 

 East Fork of Beaver Creek, Black Hills. Associated with fossils of the 

 Fort Beuton group. 



GEXUS AKKRA, 0. F. Muller. 



AKERA GLANS-ORYZA, n. sp. 







Plate 12, fig. 25. 



Shell minute, not exceeding one-fifth of an inch in length, and the 

 diameter being about twice and a half less than the length ; cylindrical 

 in form, with a very low, truncated spire, which consists of about three 

 or three and a half volutions, the outer margins of which are bordered 



