102 UNITED STATES GEOLOGICAL SUEVEY OF THE TERRITORIES. 



mens, the very fine radiating striae are readily seen, even without the aid of a 

 magnifier ; but one of them is partly covered by an exceedingly thin 

 outer layer, on which they are much more obscure. From this fact, I infer 

 that the radiating striae are probably not always readily seen on the outer 

 surface-layer of well-preserved specimens. 



This species has somewhat the appearance of the next, but may be dis- 

 tinguished at a glance by its less convex form, non-striate surface, and smooth 

 inner margins. 



Locality and position. — Moreau River, Long Lake, Butte au Gres, &c. ; 

 from the Fox Hills group, or No. 5 of the Upper Missouri Cretaceous series. 



Nucula canccllata, M. &H. 



Plate 28, figs. 13, a, b, c, d, c. 

 Kucula cancellata, Meek and Haydeu (Ap., 1856), Proceed. Acad. Nat. Sci. Philad., VIII, 85. 



Shell transversely ovate-subtrigonal, rather gibbous ; posterior or shorter 

 side obliquely truncated above, and subangular below, the middle ; anterior 

 or longer side a little more rounded at the extremity ; basal margin semi- 

 elliptic or semi-oval in outline, neatly crenulate within ; dorsum declining 

 with a gently convex outline in front of the beaks, and sloping more abruptly 

 behind ; lunule-like area behind the beaks obovate, flattened, or a little con- 

 cave, and bounded on each side by a very slight ridge; escutcheon-like area 

 in front of the beaks lance-ovate and moderately well defined; beaks rather 

 gibbous, incurved, nearly touching, and located a little nearer the middle than 

 the posterior side ; surface marked by numerous small, regular, simple, radi- 

 ating striae, which are broader than the linear depressions between, near the 

 lower border, and crossed on all parts of the valves by smaller, more irregular, 

 concentric costae, so as to form a neat subcancellate style of sculpturing; hinge 

 thick, forming an angle of near 120° at the beaks, and provided with about 

 seventeen to nineteen teeth in front, and about twelve behind the beaks, in 

 each valve of an adult. 



Length, 0.93 inch; height, 0.G5 inch; convexity, 0.51 inch. 



In this species, the substance of the shell is rather thick, and usually 

 retains its bright-pearly lustre. Where exfoliated, the surfaces of the inner 

 laminae are seen to be marked by faint radiating striae, which are also faintly 

 visible on the inner surface of the valves. Casts of the interior show that the 

 muscular and pallial impressions are not very strongly marked. The scar of 



