33 



We have great doubts of tbe identity of the western forms with I. 

 BaraUni Morton, from Alabama, and are inclined to believe they would 

 be more naturally classed under a distinct name, as formerly done by 

 Messrs. Meek & Hayden. 



Formation awl locality. In limestone of the Fort Pierre group, at Old 

 Woman's Fork of the Cheyenne River, Black Hills, Dakota, where they 

 occur very abundantly, densely packed together in the rock, and pos- 

 sessing the specific characters and the oblique sulcus in a remarkably 

 constant degree. 



MCULASID^E. 

 Genus NUCULANA. 



XUCULANA SUBEQUILATERA, 11. Sp. 

 * Plate 11, figs. 3 and 4. 



Shell of small size, transversly 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 recur- 

 ring in the outer part ; anterior end of the shell sharply rounded ; pos- 

 terior extremity acutely rounded near the extremity of the cardinal line, 

 and rapidly sloping forward 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 num- 

 ber 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. Xo sinus can be detected in the pallial line. 



The species seems to be nearly related to Nucula (Nuculanaf) equilat- 

 eralis, M. & H., but the beaks are not central as in that species, but 

 are situated 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 Itiver, 

 near French creek ; supposed to be very near the top of the Fort Pierre' 

 group. 



3 B H 



