134 UNITED STATES GEOLOGICAL SURVEY OP THE TERRITORIES 



impressions are obsolete, while in other cases they, are quite distinct, and 

 there seem to be all gradations between. In one specimen, from the same 

 locality and position, agreeing exactly in its surface- markings, the thinness 

 of the shell, and, indeed, in almost every other respect, with the above, there 

 are two impressions, with a ridge between, passing down the front side, to 

 the margin of which they impart a very peculiar appearance. This may be 

 a distinct species; but, with the material now at hand for comparison, it seems 

 probable that it is only a variety of the above. Fig. 2 e of pi. 17 represents 

 this -specimen. 



Locality and position, — Mouth of Sage Creek, on the Cheyenne; in the 

 upper part of the Fort Pierre group of the Upper Missouri Cretaceous series. 

 The specimen of this species first figured in the paper cited at the head of 

 this description was collected by Dr. Hayden from the same geological posi- 

 tion, on Sage Creek, and near the same locality. 



Lncina occidentalis, Morton. 

 Plate 17, figs. 4, a, b, c, d. 



Tellina occidentalis, Morton (1842), Jour. Acad. Nat. Sci. Philad., VIII, 210, pi. xi, fig. 3. 

 Mould of Lncina ?, Owen (1852), Report Geol. Survey Wis., Iowa, and Min., tab. vii, fig. 8. 

 Lucina occidentalism, Meek ami Hayden (Nov., 185G), Proceed. Acad. Nat. Sci. Pbilad., VIII, 272 (not 

 Lncina occidentalis, Reeve, 1850). 



Shell transversely broad-oval, rather thick, moderately convex ; anterior 

 side broadly rounded ; basal border semi-ovate in outline, the most prominent 

 part being toward the front; posterior side narrower, and subtruncated at the 

 extremity, usually having a nearly obsolete flattening, extending from near 

 the beaks, above the umbonal slopes, obliquely backward and downward to 

 the posterior extremity; dorsal border nearly straight, or faintly sinuous, and 

 declining very slightly in front of the beaks, convex, and more obliquely 

 sloping behind; beaks depressed, small, and nearly central; lunule lanceo- 

 late, small, and shallow, or somewhat excavated. Surface ornamented by 

 very distinct, rather regular, concentric lines; exfoliated specimens also 

 showing obscure radiating marks on the inner laminae. 



Length of a large specimen, 1.85 inches; height, 1.58 inches; convexity, 

 0.93 inch. 



The hinge of this species shows the two cardinal teeth to be rather 

 small in the right valve, the posterior one being a little larger than the other, 

 and faintly emarginate at the extremity. Between these two teeth there is 



