8tanton.1 VENERID.E. 109 



^Locality and position. — Five miles below the mouth of James river, 

 at the base of the Fort Benton group, or formation No. 2 of the Upper 

 Missouri Cretaceous. It was from this same locality and position that 

 the specimen first figured by Prof. Hall and the writer was obtained.' 7 



Callista (Aphrodina?) tenuis H. & M. 

 PI. xxiv, Figs. 7 and 8. 



Cytherea tenuis Hall and Meek, 1856, Mem. Am. Acad. Arts and Sci., n. s., vol. v, p. 383, 



PI. 1, Fig. 5. 

 Meretrix tenuis Meek and Harden, 1860, Proc. Acad. Nat. Sci., Phila., p. 185. 

 Dione ? tenuis Meek, 1864, Smithsonian Check-List, Invert. Foss. of N. A., p. 13. 

 Callista (Aphrodinaf) ten wis Meek, 1876, U. S. Geol. Sur. Terr., vol. ix, p. 188, PI. 5, 



Figs. 1, a-d. 



Revised description : 



" Shell thin, varying in form from subcireular to transversely ovate, 

 rather gibbous at maturity. Anterior side rather short, obliquely sub- 

 truncate above, and abruptly rounded below; base forming a semi- 

 *)vate curve, being a little more prominent before than behind the mid- 

 dle; posterior side narrowly rounded, or subtruncate; beaks moder- 

 ately prominent, somewhat gibbous in old shells, incurved, contiguous, 

 and placed nearly halfway between the middle and anterior side; lunule 

 obovate, flat, and not very distinctly defined. Surface marked by fine, 

 crowded, concentric stripe, and sometimes a few, obscure, parallel fur- 

 rows near the border. 



"Length of an adult shell (oval var.), 1 inch; height, 0.78 inch; con- 

 vexity, 0.56 inch. 



" None of our specimens expose the interior, but internal casts show 

 that the muscular impressions are faintly marked and of a rather nar- 

 row ovate form, both before and behind. The pallial line is also seen 

 to be provided with a broad, shalloAv sinus, having nearly the form of 

 an equilateral triangle. Little or nothing being known in regard to 

 the teeth of its hinge, it is, like the last, only referred doubtfully to 

 this group. By working carefully about the hinge, however, and grind- 

 ing down some of the specimens, I have nearly satisfied myself that it 

 has a small anterior lateral tooth as in Meretrix (= Cytherea) and Cal- 

 lista; and as the pallial sinus is distinctly triangular, as in some sec- 

 tions of the latter, I think there is not very much reason for doubting 

 the propriety. of placing it, at least provisionally, in the latter genus; 

 though better specimens may show it to belong to some other group. 



''Locality and position. — South fork of Cheyenne river, near the base 

 of the Black hills, in the Fort Union [Benton] group, or formation No. 2 

 of the Upper Missouri Cretaceous. The specimens first described in the 

 paper cited at the head of this description were collected from the same 

 formation on the Missouri river, 5 miles below the mouth of James river. 



