stanton.] APORRHAID.E. 143 



robust, and it is marked by strong transverse costae. It resembles the 

 Ooniobasis cleburni White of the Bear Kiver formation, excej)ting that 

 it is not quite so slender. 



APORRHAID^. 



Genus APOKRHAIS Dillwyn. 



Aporrhais (Goniocheila) castorensis Whitfield. 



PI. xxxi, Fig. 1. 



Aporrhais (Goniocheila) castorensis Whitfield, 1877, Prelim. Rept. Paleont. Black Hills, 

 p. 38 j 1880, Geol. Black Hills of Dakota, p. 427, PI. 12, Fig. 1. 



Original description : 



u 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 distinct; apical angle about 40°, but slightly variable on differ- 

 ent individuals. Body volution proportionately large and very strongly 

 angular, or even carinate, 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 carinatiou 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 pro- 

 jecting in the middle to form a short, obtuse, slightly recurved digita- 

 tion, and posteriorly extending along the spire to the base of the sec- 

 ond volution above. 



" This species somewhat resembles A. biangulata M. & H., 1 but 

 differs in the subdned 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 

 described, nor any evidence of a second digitation to the lip. The sur- 

 face markings are quite superficial, but few specimens showing them, 

 appearing quite smooth from slight exfoliation. 



" We are extremely adverse 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 collec- 

 tion of specimens, to be only a form of A. biangulata M. and 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 

 separate it under a new name. 



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

 east fork of Beaver creek, Black hills. Associated with fossils of the 

 Fort Benton group." 



* Paleontology of the United States Geological Survey of the Territories, p. 322, PI. 19, Fig. 6. 



