NATURAL SCIENCES OF PHILADELPHIA. 181 



angle of about 60 with the umbonal axis. Beaks narrow, rising somewhat 

 above the hinge, scarcely incurved, located about halfway between the middle 

 and the most prominent part of the anterior border. Surface ornamented by 

 very slightly elevated, broadly rounded, rather irregular undulations, which 

 become entirely obsolete on large specimens below the middle, find on the pos- 

 terior as well as the lower anterior regions. Attains a diameter of 3j to 4 

 inches. 



Named after Lieut. G. T. Balch, of U. S. Ordinance who discovered the 

 only specimens of the species we have seen. 



Locality and position. White River above the Mauvaises Terres. 



Inoceramus subcompressds, M. & H. Shell rhombic-oval, compressed, very 

 thin ; anterior side rounded below the beaks ; base forming a long semiovate 

 curve, the most prominent part of which is behind the middle ; posterior side 

 long, very narrowly rounded and prominent below the middle, subtruncate ob- 

 liquely forward above ; hinge of moderate length, forming an angle of about 40 

 with the umbonal axis. Beaks small, scarcely rising above the hinge, located 

 nearly over the anterior extremity. Surface ornamented by somewhat regular 

 concentric undulations. Length 2-55 inches ; height 1-T0 inches. 



Locality and position. Mouth of Judith River, formation No. 1 ? of Nebraska 

 section. 



Inoceramus avicdloids, M. & H. Shell compressed, often broad ovate or 

 subcircular when young, but becoming obliquely oval or subrhomboidal in 

 outline as it advanced in age ; substance thin and fragile. Anterior and basal 

 margins forming a broad gentle curve ; posterior extremity narrowly rounded 

 below, ascending obliquely forward, with a slightly convex outline above, and 

 meeting the hinge at an angle of about 120. Hinge margin long, straight and 

 compressed, so as to form an alate expansion behind. Beaks nearly terminal, 

 scarcely rising above the hinge, not gibbous or distinctly incurved. Surface 

 ornamented by more or less regular concentric undulations and obscure lines 

 of growth. Length from the beaks obliquely backward and downward to the 

 postero-basal edge, about 3 inches ; height from base to hinge, 2-30 inches. 



Locality and position. Little Blue River, formation No. 3. 



Anomia obliqua, M. & H. Shell thin, broad oval, subcircular, or somewhat 

 irregular, and more or less oblique; upper valve rather convex, beak nearly or 

 quite marginal, and placed nearer the anterior side, moderately gibbous ; sur- 

 face marked concentrically by fine obscure lines, and small wrinkles of growth. 

 Length about 1-32 inches ; breadth 1*16 inches. 



Locality and position. Near mouth of Niobrara River, in formation No. 3 of 

 the Nebraska section. 



Anomia subtrigonalis, M. & H. Shell subtrigonal, approaching subcircular, 

 extremely thin and fragile ; upper valve moderately convex ; anterior side sub- 

 truncate, with a slightly convex outline, rounding abruptly at its junction with 

 the ventral margin; posterior side obliquely truncate' from the beak, and very 

 narrowly rounded at its connection with the ventral border, provided with a 

 broad, oblique, rounded fold ; pallial margin nearly straight, or but slightly 

 convex ; umbo marginal and rather prominent. Lower valve nearly flat, or 

 compressed, and more irregular than the other. Surface marked by small, 

 irregular, concentric wrinkles, and very obscure lines of growth. Length 1*57 

 inch; breadth 1-14 inch. 



Locality and position. Bijou Hill, on the Missouri, formation No. 4. 



Ostrea inornata, M. & H. Shell small, narrow subovate, rather thin, 

 attached by the whole under surface of the lower valve ; beaks pointed and 

 curved usually to the left side; under valve conforming to the contour of the 

 surface to which it adhered, moderately concave, area small and narrow ; upper 

 valve rather convex, having its beak less pointed than that of the other valve ; 



I860.] 



