100 UNITED STATES GEOLOGICAL SURVEY OF THE TERRITORIES. 



concentric striae, as the species of this genus having radiating costse or stria? 

 usually retain some traces of them on casts of the interior. Its small size, and 

 decidedly compressed form, will distinguish it from all the other Upper Mis- 

 souri species yet known. 



It appears to be very similar in form and general appearance, to JV. Ray- 

 monds of M. d'Archiac, from Barns de Rennes (Bui. Geol. Soc. Franc., II, 

 2e se*r., pi. 3, fig. 16), but is proportionally a little shorter, and its muscular 

 impressions are less distinct. It also differs in having its inner margin smooth, 

 while that of N. Raymondi is described as being finely crenulate. 



Locality and position. — Yellowstone River, 150 miles from its mouth, in 

 a bed containing a blending of the fossils of the upper part of the Fort Pierre 

 and Fox Hills groups; or Nos. 4 and 5 of the Upper Missouri Cretaceous 

 series. 



IV ii c u la obsolctistriiita, M. & H. 



Plate 15, figs. 10, c, d. 

 Nucula obsolelistriata, Meek and Hayden (Nov., 1850), Proceed. Acad. Nat. Sci. Pliilad., VIII, 275. 



Shell transversely elliptic-oval, rather convex, thick; anal or shorter side 

 obliquely truncated, with a concave outline, from the beaks to posterior basal 

 extremity, which is angular or very narrowly rounded; anterior extremity 

 more obtusely subangular; base forming a semi-ovate curve, the most 

 prominent part of which is generally a little in front of the middle; 

 cardinal edges forming at the beaks an angle of about 123°; beak small, 

 depressed, oblique, placed a little behind the middle; surface marked by fine 

 lines of growth, and sometimes a few very small, obscure, concentric wrinkles; 

 lunule-like area narrow-obovate, deeply impressed, and distinctly defined on 

 each side by an obtuse ridge. 



Length, about 1.20 inches; height, 0.81 inch; convexity 0.60 inch. 



The cardinal edge is thinnest near the beaks, from which point it grad- 

 ually thickens toward the extremities, especially in front of the beaks. The 

 pit for the reception of the ligament is acutely triangular and oblique. 

 The posterior muscular impression is narrow-ovate, deeply excavated, and 

 placed close up under the extremity of the hinge; while the anterior one is 

 still narrower, quite shallow, and located very close up under the anterior end 

 of the same. There are about thirty anterior hinge-teeth in each valve of 

 adult shells, and nearly half as many behind the beaks, all of which are curved 



