180 PROCEEDINGS OF THE ACADEMY OF 



Locality and position. North branch of Cheyenne River, near Black Hills, 

 formation No. 5. 



Avicula subsibbosa, M. k H. Shell (left valve) obliquely rhombic-oval, 

 or ovate, moderately gibbous; anterior margin contracted, or a little concave 

 in outline just below the wing, from which point it descends obliquely back- 

 ward, with a broad, gently convex sweep, into the base ; posterior border rather 

 broadly rounded below, distinctly sinuous under the wing above ; hinge line 

 Straight, a little less than the height of the shell. Anterior wing forming an 

 equilateral triangle, compressed, and rather distinct from the umbo; posterior 

 wing having the form of a very inequilateral triangle, the posterior side of 

 which is much the shortest, compressed, moderately distinct from the more 

 gibbous part of the valve, forming an angle of about 50 at the extremity ; beak 

 small, slightly elevated above the hinge, gibbous, located a little less than one- 

 third the length of the hinge, behind the anterior extremity ; posterior muscu- 

 lar scar large, oval or ovate, and located a little above the middle. Height 

 1-40 inches ; length, measuring from the postero-basal extremity obliquely 

 forward and upward to the point of the beak, 1*72 inches ; length of hinge 

 1-32 inches. 



This species resembles A. linguiformis, Evans & Shumard, but is much broad- 

 er and less oblique, while its postero-basal margin is more broadly rounded. 

 Our specimen is a cast, and does not show the surface-markings, excepting on 

 the anterior wing, where the marks of growth are rather distinct and subim- 

 bricating. 



Locality and positio?i. Long Lake, above Fort Pierre, formation No. 5. 



Inoceramus cuneatus, M. & H. Shell oblong-ovate, moderately gibbous in 

 the umbonal and anterior regions, very nearly or quite equivalve, rather thin ; 

 buccal side descending from the beaks ai first, almost at right angles to the 

 hinge, after which it gradually curves obliquely backward and downward, so 

 as to pass by a graceful sweep into the base ; posterior side long, compressed, 

 broader than the other extremity, and regularly rounded ; ventral border form- 

 ing a semiovate curve, the most prominent part of which is behind the middle ; 

 hinge very long, and nearly straight. Beaks very nearly terminal, or located 

 almost directly over the anterior border, oblique, rising little above the hinge, 

 equal, and but slightly incurved. Surface marked by rather distinct, more or 

 less regular undulations. Length 3-90 inches; height 2-75 inches ; convexity 

 2 inches. 



Locality and position. Yellow Stone River, 150 miles above the mouth, ia 

 beds containing a blending of the fossils of formations Nos. 4 and 5. 



Inoceramds Vanuxemi, M. & H. Shell large, subcircular or broad oval, 

 equivalve, and much compressed ; anterior margin rounded ; base forming a 

 nearly semicircular curve, being a little more prominent behind than in front ; 

 posterior side longer and wider than the other, broadly rounded orsubtruncate; 

 hinge (of moderate length ?) straight, and forming an angle of about 70 with 

 the axis of the umbones. Beaks small, compressed, scarcely rising above the 

 hinge, not distinctly incurved, located a little in advance of the middle. Sur- 

 face ornamented by regular, distinct, angular, but not very prominent concen- 

 tric undulations, which are separated by rather shallow depressions. Length 

 of the largest specimen we have seen, 10 inches ; height of do. 9 inches. 



Locality and position. White River above the Bad Lands, in upper part of 

 formation No. 4. 



Inoceramus Balchii, M. & H. Shell large, subquadrate, or broad oblong- 

 oval, much compressed ; anterior side truncate obliquely forward above, at an 

 angle of about 115 with the hinge, rounding into the base below; ventral 

 margin forming a broad curve, the most prominent part of which is a little 

 behind the middle ; posterior side longer and wider than the other, broadly 

 rounded, (sometimes subtruncate above ?) ; hinge line rather long, forming an 



[May, 



