CRETACEOUS FOSSILS. 407 



tained, has been faintly concentrically striate, and there is also evidence on 

 the exfoliated surface of fine obscure radiating lines. 



On the internal casts the muscular imprints are distinctly marked, and 

 of rather large size, the posterior scars oval and placed close to the cardinal 

 line, and the anterior imprints of nearly the same size and form, but sub- 

 marginal ; pedal scars elongate oval, very much smaller than the posterior 

 adductors, and placed a little in advance of them, and close beneath the 

 hinge plate Teeth numerous; evidence of about twenty- four can be seen 

 on the posterior side of the beaks, and of about ten or eleven on the ante- 

 rior side. 



The specimens in hand are imperfect and much exfoliated, so that tlu? 

 entire features cannot be determined. The}^ do not agree in all respects with 

 the species to which we have referred them, being a little broader posteriorly 

 and rather more excavated in front of the beaks anteriorly; still it is too 

 nearly related to be considered distinct. 



Formation and locality. — In limestones of the Fort Pierr^ Group, on the 

 Cheyenne River, ten or fifteen miles west of the mouth of Beaver Creek, 



Black Hills. 



Genus NUCULANA Link. 



NUCULANA BISULCATA. 



Plate 11, fig. 7. 



Leda bisulcata M. & H.. Proc. Acad. Nat. Sci. Phil., 1861, p. 440. 



Xuculana bisulcata Meek, Smithsonian Check List Invert. Cretaceous Foss., p. 8. 



Nuculana bisulcata Meek, U. S. Geol. Surv. Terr., p. 104, PI. 15, fig. 4. 



Shell quite small, not exceeding three-eighths of an inch in length, the 

 extreme height rather less than half the length, subovate or subelliptical in 

 outline; anterior end largest and broadly rounded; posterior end narrower 

 and compressed, subangular below; basal line broadly curved and slightly 

 constricted behind the middle; beaks full and strong, rather more than one- 

 third of the length from the anterior end; cardinal margins sloping gradu- 

 ally from the beaks, the posterior side sometimes very slightly concave. 



Surface of the valves convex, and marked by fine, even, concentric lines. 



The specimens of this species are quite similar to X. suheqi dial era herein 

 described, but are not more than one-third as long, and the cardinal line is 



