224 UNITED STATES GEOLOGICAL SUEVEY OE THE TERRITORIES. 



valve, with portions of the inner laminae of the shell adhering. On the sur- 

 faces of these, faint traces of radiating strire are visible ; but they appear to be 

 due to the structure of the shell, and not connected with surface-markings. 



The muscular impressions are shallow: the anterior one is narrow-oval e, 

 placed near the anterior margin, and stands with its upper end slightly 

 inclined backward ; while the posterior is broad-ovate, located at the back 

 extremity of the dorsal edge, and inclined obliquely forward.* The sinus of 

 the pallial impression is rather broad, but a little longer than wide, and 

 directed somewhat obliquely forward and upward, so as not to be exactly 

 parallel with the horizontal axis of the shell. It appears to be rounded at 

 the extremity, and extends about two-fifths of the distance from the anal 

 margin toward the middle of the valves. 



The narrow-elliptic and compressed form of this species, together with 

 its depressed beaks, will distinguish it from all of the otherwise similar shells 

 known to occur in the Nebraska rocks. Since first describing it, I have 

 worked away the matrix from the hinge, and find it presents, as near as can 

 be determined, more the characters of Thracia than those of Tell 'na. Until 

 better specimens can be examined, however, there must remain some doubts 

 in regard to its generic relations. 



Locality and position. — Mouth of Judith River, Idaho Territory, on the 

 Missouri ; in a sandstone of Cretaceous age, now known to hold a position 

 at the horizon of the top of the Fox Hills group. 



Thracia gracilis, M. & H. 



Plate 39, figs. C, a, b. 



Tellina gracilis, Meek aud Hayden (April, 1856), Proceed. Acad. Nat. Sci. Philad., VIII, 82. 

 Thracia t gracilis, Meek aud Hayden (Nov., 1856), ib., 284. 



Shell transversely elliptical-ovate, moderately compressed, extremely 

 thin and fragile ; anterior end rounded ; posterior half rather contracted, 

 vertically subtruncated, and apparently a little gaping at the extremity, pro- 

 vided with an obscure ridge, which passes from the beaks obliquely backward 

 and downward toward the postero-basal margin ; ventral outline forming a 

 broad, semi-ovate, or semi-elliptic curve; dorsum sloping slightly and convex 

 in front of the beaks, concave and declining somewhat behind them; beaks 



* Unfortunately, neither of these impressions is represented exactly right in our figures, heing 

 made to appear too obtuse above. The anterior one, especially, should be pointed, instead of rounded, 

 at the upper end. 



