182 PROCEEDINGS OF THE ACADEMY OF 



surface smooth, or only marked by very obscure lines of growth, with sometimes 

 a few very small, irregular, nearly obsolete radiating wrinkles near the lower 

 border. Length about 1-40 inches; breadth 0-87 inch. 



Locality and position. Great Bend of the Missouri, below Fort Pierre lower 

 part of No. 4, Nebraska section. 



JURASSIC SPECIES. 

 CONCHIFERA. 



Pholadomya humilis, M. & H. Shell transversely oblong-oval, ventricose ; 

 posterior end rounded, and more or less gaping; base nearly straight along the 

 middle ; anterior end very short, narrowly rounded below the beaks ; dorsum 

 nearly parallel with the base, slightly concave in outline ; escutcheon lanceo- 

 late, and bounded by an obscure angle on each side ; beaks depressed, gib- 

 bous, incurved, and located in advance of the middle; surface ornamented by 

 small, regular, concentric wrinkles, crossed by a few raised lines, or obscure, 

 distant, radiating costae, extending from the back part of the beaks, to the pos- 

 terior, and postero-basal margins. Length about 1-06 inch : height 0-52 inch ; 

 breadth 0-52 inch. 



Locality and position. Lower Jurassic series, at the south-west base of the 

 Black Hills. 



Myacites Nebrascensis, M. & H. Shell elongate, subelliptical, rather convex ; 

 extremities narrowly rounded, the posterior end being sometimes apparently 

 obliquely subtruncate, and more or less gaping above ; base nearly straight, or 

 very slightly sinuous along the middle, rounding up gradually towards the 

 ends; dorsum behind the beaks concave in outline ; posterior umbonal slopes 

 gibbous, or prominently rounded ; antero- ventral region a little compressed, 

 or contracted from near the middle of the base obliquely forward and upward ; 

 beaks moderately elevated, gibbous, incurved, and located near the anterior 

 end ; surface ornamented by concentric striae, and small, very obscure, irregu- 

 lar parallel wrinkles. Length about 1-43 inch ; height 0-69 inch ; breadth 0-59 

 inch. 



Locality and position. South-west base Black Hills. Jurassic. 



Thracia? sublevis, M. & H. Shell narrow oblong-oval, rather compressed ; 

 anterior end narrowly rounded ; base nearly straight along the middle, round- 

 ing up toward the ends ; posterior side longer than the other, rounded or 

 slightly truncate, and apparently gaping a little at the extremity; dorsal bor- 

 der concave in outline, and nearly horizontal behind the beaks, declining more 

 abruptly in front; beaks moderately elevated, the right one being usually a 

 little higher than the other, located in advance of the middle ; posterior um- 

 bonal slopes prominently rounded ; surface concentrically striate ; hinge and 

 interior unknown. Length 1-17 inch; height 060 inch ; breadth about 0-32 

 inch. 



Locality and position. Near the middle of the Jurassic deposits at the south- 

 west base of the Black Hills. 



Thracia? arcuata, M. & H. Shell small, transversely subovate, more or 

 less arcuate, moderately convex ; extremities rather narrowly rounded, and a 

 little gaping; cardinal margin sloping from the beaks, anterior slope more 

 abrupt than the other ; beaks rather elevated and unequal, that of the right 

 valve being higher than the other, located in advance of the middle ; posterior 

 and anterior umbonal slopes prominent; sides of the valves flattened or slightly 

 concave in the central region near the base; surface of cast retaining small 

 concentric marks of growth ; hinge and interior unknown. Length, 62 inch ; 

 height, 0-37 inch ; thickness or convexity. 0-23 inch. 



Locality and position. Same as last. 



Cardium Shumardi, M. & H. Shell small, subcircular, rather gibbous ; an- 



[May, 



