102 



PALAEONTOLOGY OF THE UPPER MISSOURI. 



under Thracia, while others adopt it as a distinct genus. Whether we regard the 

 type upon which it was founded as generically or subgenerically distinct, however, 

 Bellevue's name should probably not be retained, since it was used by Brisson, in 

 1760, for a genus of birds, now adopted by ornithologists. 



Thracia? sufolaevis. 



(PLATE IV, Fig. 4, and 4a.) 



Thracia? sublxvis, MEEK & HAYDEN, Proceed. Acad. Nat. Sei. Phila. May, 1860, 182; ib. Oct. I860, 418. 

 Shell elongate, or narrow oblong-oral, rather compressed. Anterior end narrowly rounded ; base nearly straight 

 along the middle, rounding up toward the ends ; posterior side longer than the other, rounded, or slightly truncate, 

 and apparently gaping a little at the extremity ; dorsal border straight or 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 umbonal slopes prominently rounded. 

 Surface concentrically striate. 



Length, 1.19 inch ; height, 0.63 inch ; breadth, about 0.32 inch. 



Not having seen the hinge or the interior of this shell, we are left in doubt 

 respecting its true relations, and merely place it provisionally in the above genus. 

 Our specimens are all casts, but some of them retain traces of concentric lines, 

 which appear to be the only kind of surface markings it had. Behind the beaks 

 there is a narrow area or escutcheon, apparently formed by the thickening or in- 

 flection of the cardinal margin ; this, however, may exist only on the internal cast. 



Locality and position. Near middle of the Jurassic beds, at the southwest base 

 of the Black Hills. (Type No. 197.) 



Thracia? arcuata. 



(PLATE IV, Fig. 8.) 

 Thracia? arcuata, MEEK & HATDEN, Proceed. Acad. Nat. Sci. Phila. May, 1860, 182 ; ib. Oct. I860, 418. 



Shell small, longitudinally snbovate, more or less arcuate, moderately convex. Extremities rather narrowly 

 rounded, and gaping a little. 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 compressed or slightly concave in 

 the central region, near the base. Surface of cast retaining small concentric marks of growth. (Hinge and 

 interior unknown.) 



Length, 0.62 inch ; height, 0.34 inch ; thickness, 0.23 inch. 



This shell is more nearly related to the last than to any of the other species yet 

 known from the rocks of the upper Missouri country, but differs in being more gib- 

 bous, and in having the valves more compressed or concave near the middle of the 

 base, which imparts an arcuate outline to the ventral border. Its dorsal margin is 

 also much less nearly horizontal, in consequence of the greater elevation of its beaks. 

 Like the last, it has along the cardinal border of the internal cast, behind the beaks, 

 a narrow circumscribed area or escutcheon, bounded by an obscure angle along each 

 side. Knowing nothing of the nature of its hinge, or pallia! line, we cannot deter- 

 mine its generic relations with much confidence. 



Locality and position. Same as last. (No. 211.) 



Genus PHOLADOMYA, SOWERBY. 



Synon. Pholadomya, G. B. SOWEBBY, Genera Shells, 1823, fasc. 19. LATH. Fam. Nat. 1825. DEFRANCE, Diet. Soi. 

 Nat. t. XXXIX, 1826, 535. J. SOWERBY, Min. Conch. 1827, tab. 545. DESHAYES, Encyc. Meth. Ill, 1830, 



