INVERTEBRATE PALEONTOLOGY. 135 



a little larger pit, and farther back a smaller one, for the reception of the two 

 cardinal teeth aider the beak of the left valve. Each valve has one small, 

 obtuse, anterior, lateral tooth, situated some distance in advance of the beaks x 

 with apparently very faint traces of a remote posterior lateral in one or both 

 valves. 



The posterior muscular impression is shallow, subquadrate in form, and 

 usually bounded in front by a faint linear ridge, extending obliquely up 

 toward the beaks. The upper part of the anterior muscular impression has 

 the same form, while its prolonged portion below is slender, a little arcuate, 

 rather long, and directed obliquely downward toward the middle of the basal 

 margin. Just above this impression of the anterior adductor, the small, oval 

 scar of the pedal muscle is seen quite detached from it. 



It is with some doubt and hesitation that I have identified this shell 

 with Tellina occidentals of Morton. Well-preserved specimens of it are 

 certainly quite unlike Dr. Morton's figures, though when the shell is partly 

 worn away along the dorsal side in front of, and behind the beaks, as is the 

 case with some of our specimens, they present almost exactly the same 

 appearance. In addition to this, it is evident Dr. Morton's figures were drawn 

 from a worn and otherwise imperfect specimen, giving an incorrect idea of 

 its original form; while the species here under consideration appears to have 

 a wide geographical range in Nebraska, and is nearer like the specimen 

 figured by Dr. Morton than any other shell yet known from the rocks of 

 that region. 



Locality and position. — Mouth of Milk River, and on Cheyenne River; 

 in the upper part of the Fort Pierre group. The specimens from the latter 

 localities have a rather shorter anterior muscular impression, and present 

 some other slight differences, which may possibly be of specific importance ; 

 but I think that they more probably merely belong to a variety of the same. 

 It also occurs on the Moreau River, where it seems to range up into the Fox 

 Hills group, or formation No. 5 of the Upper Missouri Cretaceous. 



Lncina occidental^, var. ventricosa, M. & H. 



Plate 17, fig. 3, a, b, c. 

 Liwina ventricosa, Heek and Hayden (1800), Proceed. Acad. Nat. Sci. Philad., VIII, 4-27. 



Shell very broad-ovate, or subcircular, rather thin, compressed or 

 moderately convex: anterior side broadly rounded: basal border smooth 



