110 UNITED STATES GEOLOGICAL SUKVEY OP THE TERRITORIES. 



Voldia sciliilu, M. & H. 



Plate 28, fig. 9. 



Nucula scilida, Meek and Hayden (April, 1856), Proceed. Acad. Nat. Sci., Pliilad., VIII, 84. 



Leda scititla, Meek and Haydon (I860), ib., XII, 185. 



Leda (Yoldia) scititla, Meek and Hayden (October, 1860), ib., 428. 



Shell transversely subovate, gibbous in the central and umbonal regions ; 

 anterior extremity rather narrowly rounded; posterior side narrower and 

 more compressed, subangular or very narrowly round in outline, the most 

 prominent part being above the middle ; base forming a semi-ovate curve, 

 sometimes very slightly sinuous near the middle ; dorsum declining from the 

 beaks; cardinal border of each valve having a distinct marginal groove 

 behind the beaks, which forms, when the valves are united, a lanceolate, 

 escutcheon-like depression ; beaks rather obtuse, not oblique, placed a little 

 in advance of the middle; surface marked by regular, fine, distinct, concentric 

 lines, which are nearly equal to the grooves between, and more strongly 

 defined on the middle than toward the extremities of the valves. 



Length, 0,40 inch ; height, 0.21 inch ; convexity, 0.20 inch. 



This species resembles in form and general appearance Yoldia ventricosa 

 (z= Nucula ventricosa of Hall and Meek), but differs in being a much larger 

 and less ventricose shell. It is also less prominently rounded on the ventral 

 border, and its beaks less nearly central, while its concentric markings are 

 proportionally smaller. 



Our shell is also similar to Leda Maries, d'Orbigny (Palseont. Fr., Ill, 

 pi. 301, figs. 4-6), a Gait species, but is much more gibbous, and has more 

 elevated beaks, which are located nearer the middle. 



Some internal casts of our species show that its pallial line had a rather 

 distinct sinus, though the sinus is not deeper than we sometimes see in the 

 genus Leda. As the form of the shell, however, agrees more nearly with 

 Yoldia, I have placed it in that group. 



Locality and jwsition. — Moreau River and Long Lake; in the Fox Hills 

 group, or No. h of the Nebraska Cretaceous series. We also have it from 

 beds on Yellowstone River, containing a mingling of species usually found in 

 the upper part of the Fort Pierre group and the Fox Hills beds. 



