106 UNITED STATES GEOLOGICAL SUEVEY OF THE TERRITORIES. 



Length, 0.78 inch; height, 0.45 inch; convexity, 0.36 inch. 



I have not yet seen the interior of this shell, and consequently have no 

 means of determining whether or not its pallia! line is sinuate. It is mani- 

 fest, however, that it cannot be properly retained in the genus Nucula as 

 restricted and understood by conchologists ; and from its analogy, externally, 

 to the recent Nuculana emarginata, Lamk., it most probably belongs to that 

 genus, though it differs from the most characteristic form of the same. I 

 know of no other species in Upper Missouri rocks with which it is liable 

 to be confounded. 



Locality and position. — Great Bend of the Missouri, below Fort Pierre; 

 from the lower part of the Fort Pierre group, or No. 4 of the Cretaceous 

 series. 



Nuculana? equilatcralis, M. & H. 

 Plate 15, figs. 7, a, b. 

 Nucula equilatcralis, Meek and Hayden (Ap., 1856), Proceed. Acad. Nat. Sci. Philad., VIII, 84. 



My knowledge of this species is entirely derived from the study of 

 internal casts. As nearly as can be determined from these, it may be charac- 

 terized as follows: transversely elongate-subelliptical, much depressed, mod- 

 erately convex in the central region, and narrow at the extremities ; basal 

 margin forming a regular, but very gentle curve, apparently not crenulate 

 within; hinge-line sloping slightly, and nearly equally, from the beaks toward 

 the extremities, provided in front and rear of the beaks with about sixteen 

 teeth in each valve ; beaks rather depressed, and almost exactly central ; 

 surface unknown. 



Length, 0.64 inch; convexity, 0.21 inch; height, 0.25 to 0.27 inch. 



Until the pallial line of this species can be more clearly seen, it will be 

 impossible to determine whether it is Nuculana or a Yoldia* Its form, 

 however, would exclude it from Nucula, as that genus is now understood 

 by conchologists. I know of no other species in the Upper Missouri rocks 

 with which it is liable to be confounded ; all of its associates resembling it in 

 other respects being inequilateral. Excepting in this character, it would 

 seem to be very similar in form to Yoldia Evansi. 



Locality and position. — Moreau River; in Fox Hills beds, or formation 

 No. 4 of the Cretaceous series. 



* Iu the only specimen yet seeu, its pallial line looks as if it were simple, as represented in the 

 figure ; but it may have a small sinus; or possibly a more decided one, and have to be called Yoldia 



equilatcralis. 



