408 GEOLOGY OF THE BLACK HILLS. 



much less concave behind the beaks, and the beaks nearer the anterior end 

 of the shell and less prominent. 



Formation and locality. — In limestone of Cretaceous age, on the Chey- 

 enne River, near Box Elder Creek, and on the east fork of Beaver Creek, 

 Black Hills. 



NUCULANA SUBEQUILATERA. 



Plate 11, figs. 3, 4. 

 Nuculana subequilatera Whitf., Prelim. Rept. Pal. Black Hills, 1877, p. 32. 



Shell of small size, transversely elongate-subovate, with large, promi- 

 nent, suberect beaks, which are placed a little in advance of the middle of 

 the length, and are slightly incurved, but comparatively distant ; valves 

 convex, as seen on the casts, ventricose in the middle and below the beaks, 

 and compressed toward the extremities, more particularly so posteriorly ; 

 hinge line abruptly bent between the beaks, the opposite sides forming an 

 angle of about 140°, the posterior side slightly recurving in the outer part; 

 anterior end of the shell sharply rounded ; posterior extremity acutely 

 rounded near the extremity of the cardinal line, and rapidly sloping for- 

 ward below to the basal margin, which is broadly curved and almost 

 straightened in the middle. 



Surface of the shell unknown, hinge plate marked by a large number 

 of comparatively long, slender teeth, minute in the middle and gradually 

 increasing in size and length outwardly; about twenty may, with difficulty, 

 be counted on the posterior side of the beaks and about eighteen on the 

 shorter side. Cartilage pit small ; muscular imprints faintly marked. No 

 sinus can de detected in the pallial line. 



The species seems to be nearly related to Nucula (Nuculana f) equilater- 

 alis M. & H , but the beaks are not central as in that species, but are situ- 

 ated at about two-fifths of the length from the anterior end, and the teeth 

 are decidedly not obtuse, but on the contrary are very slender, as seen on 

 the casts. 



Formation and locality. — In Cretaceous rocks on the Cheyenne River 7 

 near French Creek, Black Hills ; supposed to be very near the top of the 

 Fort Pierre Group. 



