274 [November, 



there are any posterior lateral teeth they must be small and remote from the 

 beaks. At the anal extremity of each valve there is one shallow muscular im- 

 pression, of an oval or subquadrate form, with a faint slender ridge passing 

 from its inner side obliquely up towards the beaks. At the buccal end 

 there is in each valve one long shallow, arcuate muscular impression, with its 

 narrower end downwards, and directed a little back, so that more than half of 

 its lower part is detached from the pallial impression. Immediately behind the 

 upper end of this, a second very small circular muscular impression may be 

 seen in each valve. The ligament was chiefly external, and occupied a narrow 

 lanceolate depression behind the beaks ; a portion of it, however, was partly 

 internal, being deeply seated between the beaks. Exfoliated specimens show 

 on the internal laminae of the shell, faint radiating lines, which are never visible 

 on the surface. 



From the foregoing description it will be seen this species unites characters 

 belonging to two genera. Its form and general appearance, as well as its hinge, 

 and elongated anterior muscular impression, partly detached from the pallial 

 line, are all characters that would place it in the genus Lucina; while its double 

 anterior muscular impressions indicate an affinity to the genus Corbis of Cuvier. 



We are in doubt whether or not this is the shell figured by Dr. Morton under 

 the name of Tellina occidentalis. Although certainly not very nearly like his 

 figure, it resembles it more than any other shell we have seen in all the Nebraska 

 collections. It cannot be a Tellina, as the pallial impression is unquestionably 

 simple. 



Locality and position. Near Milk River, and eighty miles above on the Mis- 

 souri, also on the Yellowstone and Moreau rivers. Formation No. 4 of present 

 section. 



Hettangia Americana. 



Shell rather thick, ovate or subrhomboidal, compressed ; posterior end much 

 broader than the anterior, obliquely subtruncate and gaping ; buccal side long, 

 rostrated and closed, very narrowly rounded at the extremity ; base forming an 

 elliptical curve, excepting near the anterior end, where it is slightly contracted ; 

 dorsal border elevated in the umbonal region, concave in front and convex 

 behind ; beaks small, approximate, located a little behind the middle, surface 

 marked with rather faint lines of growth, and sometimes, near the border, with 

 small obscure concentric wrinkles. Length 2'63 inch; height l-VG inch; 

 breadth 1-25 inch. 



We have but one specimen of this species showing the hinge. This is a left 

 valve, in which there is one strong cardinal tooth located very slightly in ad- 

 vance of the point of the beak. Immediately behind this tooth, and directly 

 under the beak, there is a distinct pit, from which a shallow depression passes 

 round the upper side of the tooth. This pit was doubtless occupied by a strong 

 tooth in the other valve. The posterior lateral tooth is prominent, obtuse and 

 compressed, while the cardinal edge between it and the depression under the 

 beak is somewhat callous. 



This shell is closely related to two or three of the species figured by Terquem 

 in the Bulletin of the Geological Society of France, (2d ser. t. x. pi. 1, 2,) but is 

 relatively higher in the umbonal region, and more distinctly rostrated in front 

 than any of them. It is also more strongly truncate behind than any of Ter- 

 quem's species, though it varied somewhat with age in this character. 



It is an interesting fact that this, the first species of the genus recognized in 

 this country, is found associated with a small Baculite, while all its known con- 

 geners in the Old World are peculiar to the Lias. 



Locality and position. No. I. Mouth of Judith River. 



CaRDIUM SPECIOStJM. 



Shell circular or round oval, higher than long, gibbous in the central and 

 umbonal regions ; buccal border a little more rounded than the anal ; base regu- 

 larly rounded ; beaks elevated, slender, pointed, incurved and nearly central, 



