(.'RETAC'KOI'S I'OSSII.S. 427 



APORltlTAIS (GONIOCUEILA) CASTORENSIS. 



Plate 12, ii;;. 1. 



Aporrhais (Goniocheila) castorensis Wbitf., Piclim. Rei)t. Pal. Black Ilills, 1877, p. 38. 



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

 flattened or very slightly ventricose 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 dis- 

 tinct ; apical angle about 40°, but slighty variable on different individuals, 

 liody volution proportionally large and very strongly angular, or even car- 

 inate, along the middle, flattened or slightly 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, 

 strongl}^ 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. hiangulata M. & H. (Paleontology 

 of the United States Geological Survey of the Territories, p. 322, PI. 19, 

 Fig. 6), but differs in the subdued character of the lower carination and in 

 the strongly uniangular form of the body volution. Among a number of 

 specimens none show the posterior canal extending above the point de- 

 scribed, 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. hiangukda M. & H., above referred to, but the single 

 strong carination and the fact that it comes from a lower geological horizon 

 and distant locality has induced us to separate it under a new name. 



Formation and locality. — In ferruginous sandy limestone on the east 

 fork of Beaver Creek, Black Hills. Associated with fossils of the Fort 

 Benton Group. 



