2G PROCEEDINGS OF THE ACADEMY OF 



LAMELLIBRANCHIA TA. 

 Genus INOCERAMUS, Sowerby. 



INOCERAMUS UNDABUNDUS. 



Shell (left valve) obliquely rhombic-subovate, gibbous, anterior side very 

 short, obliquely truncated from the beak above, and rounding into the lono- 

 antero-basal margin ; base very prominent, and abruptly rounded a little 

 behind the middle, from which point its margin ascends obliquely forward 

 with a gently convex outline ; posterior side broadly rounded or subtruncate ; 

 dorsal outline sloping from the beaks at an angle of about 90 ; hinge appa- 

 rently short; beak moderately prominent, incurved, and directed a little 

 towards the front ; umbonal axis ranging at an angle of about 70 with the 

 hinge line ; surface ornamented by regular, distinct, concentric undulations, 

 which are (on the cast) subangular, and separated h* shallow rounded depres- 

 sions ; shell structure coarsely fibrous near the fnnge. (Right valve un- 

 known.) 



Height from the most prominent part of the base to the hinge, 3 inches ; 

 length at right angles to height, 3 inches ; convexity, 1*84 inches. 



The peculiar obliquely rhombic outline, rather gibbous form, and regular 

 undulations of this shell, will readily distinguish it from any other species 

 known to us. It is true, bcth the following species have the corresponding 

 valve more gibbous than this, but in those the greatest convexity is in the 

 umbonal region, while in this it is near the middle of the valve. In all other 

 respects they are remarkably unlike. 



Locality and position. Chippewa Point, near Fort Benton, on the Missouri 

 River, from beds supposed to hold the position of Fort Benton Group of the 

 Nebraska Cretaceous section. 



INOCERAMUS EXOGYROIDES. 



Shell large ; left valve subcircular, its height being a little greater than its 

 length from the anterior to the posterior side, very gibbous ; buccal and anal 

 margins rounded, and forming with the base about three-fourths of a circle ; 

 cardinal border somewhat arched ; beak large, elevated, gibbous, distinctly 

 involuted and directed obliquely forward, so as to bring its point near the 

 anterior margin ; surface of cast smooth, or marked by obscure concentric- 

 folds. (Right valve unknown.) 



Length from anterior to posterior margin, 5 inches; height, 5*50 inches; 

 convexity near 3 inches. 



We have not yet seen the right valve of this species, but judging from the 

 gibbous character and incurved beak of the left, it will probably be found to 

 be much more compressed, so as to make the shell very distinctly inequivalve. 

 The laterally curved beak and general form of the left valve give it much the 

 appearance of some species of Exogip-a, when viewed on the inner side. Its 

 aperture is transversely oval, the height being to the length about as four to 

 five. Remaining portions of the shell about the hinge show it to have been 

 rather thick and distinctly fibrous. 



This species differs from an analogous form described by us from the same 

 position ? (and from near the same locality) under the name of I. tanbonatus, 

 in being much more depressed, and in having its beak considerably less ele- 

 vated, as well as directed much more obliquely forward. In 7. umbonatus 

 (some fine specimens of which were broxight in with the form under considera- 

 tion) the umbo of the left valve rises near one-half the entire height of the 

 shell above the hinge, while in the species we are here describing it extends 

 less than one-third the height of the shell above. The length of the valve 

 from the anterior to the posterior side is distinctly greater in proportion to its 

 height than in I. umbonatus, while the corresponding valve of the latter shell 

 is much more gibbous. We have before us a series of specimens belonging 

 to each of these forms, and find no difficulty whatever in separating them. 



[Feb. 



