NATURAL SCIENCES OF PHILADELPHIA. 311 



very nearly circular near the smaller end, and slightly oval towards the 

 aperture ; sides diverging from the apex at an angle of 8 ; septa distinctly 

 concave on the anterior sides, separated by spaces equal to one-fifth their own 

 greater diameter ; siphuncle rounded, nearly, but not quite central, a little 

 less than one sixth the diameter of the shell ; surface apparently smooth. 

 Locality and position. East side Ruby Valley, lat. 40' north, long. 115 

 20' west. Probably lower carboniferous. 



o 



JURASSIC SPECIES. 

 CONCHIFERA. 



Ostrea Engelmanni, Meek. The collection contains only upper valves of 

 this species, all of which are much compressed, rather thin, and subovate, 

 more or less irregular in form. Beak distinctly truncate and provided with 

 a broad but short area. Surface ornamented by from seven or eight, to aboiit 

 fifteen irregular, moderately distinct, rather rounded, radiating plications, 

 that do not usually extend, upon the umbo, but become quite distinct at 

 the border, which is usually thin. Lines of growth regular, and moderately 

 well defined, but not imbricating. Muscular scar rather large, ovate and 

 distinct. Length (of the largest specimen) 3-50 inches ; breadth 3 inches. 



Locality and position. Jurassic beds at Red Buttes, on the North Platte, 

 lat. 42 50', long. 106 40' west. 



Pecten bellistriata, Meek. Shell of medium size, subcircular, sometimes 

 wider than long, thin, compressed, nearly or quite equivalve ; hinge straight 

 and very short ; posterior wing small or nearly obsolete, obliquely truncate ; 

 anterior wing small, vertically truncate at the extremity, and in the right 

 valve separated from the margin below, by a distinct more or less angular 

 sinus, from which a shallow flat groove, extends obliquely to the beak ; beaks 

 of both valves small, and rather compressed ; surface ornamented by numerous 

 fine, arched, bifurcating, stria?, crossed by extremely small, closely arranged 

 concentric bines, which are often nearly obsolete on the radiating striae over 

 the more convex portions of the valves, but quite distinct in the slender de- 

 pressions between, to which they impart a punctate appearance. Length 

 (broad variety) 2-26 inches ; breadth, 2-65 inches ; convexity 0*64 inch. 



Locality and position. Same as last. 



GASTEROPODA. 



Dextalium ? subquadratum, Meek. Shell small, thin, slender, regularly and 

 slightly arcuate, very gradually tapering, flattened or a little concave on four 

 sides so as to present a subquadrangular section, the angles being a little 

 rounded ; section of internal cavity, circular ; surface apparently without lon- 

 gitudinal striae or marks of growth. Length about one inch ; diameter at 

 larger end 0"05 inch ; do. at the smaller extremity 0*02. 



Locality and position. Jurassic beds on the North Platte, at Red Buttes, lat. 

 42 50' north, long. 106 40' west. 



CRETACEOUS SPECIES. 

 CONCHIFERA. 



Anomia concentrica, Meek. Shell small, thin, subcircular or transversely 

 a little oval ; lateral extremities nearly equally rounded ; cardinal margin 

 rather straight or but slightly arched ; beak very small, central, compressed, 

 marginal, not projecting beyond the cardinal border; surface of upper valve, 

 ornamented by moderately distinct regular, concentric, undulations, and much 

 smaller obscure lines of growth. Transverse diameter 0*64 inch; length 

 from hinge to the opposite margin 0-50 inch. 



Locality and position. Near Bear River, on Sulphur Creek, lat. 41 12', 

 long. 110 50', in whitish sandstone, with an oyster very similar to O. glabra, 



I860.] 



