188 UNITED STATES GEOLOGICAL SURVEY OF TOE TERRITORIES. 



specific distinctions, were there not every gradation of form between them, 

 and all agreeing in every other respect. The prevailing form is broad-oval, 

 approaching ovate. 



As in the other cases, we bad referred this species to the genus Meretrix 

 (— : Cyther&t), from external characters, not having seen the hinge. Its deep 

 angular pallial sinus, however, shows that it must be removed from that 

 genus. It is evidently more nearly related, to sections of CaMista, as here 

 defined, though I am hardly satisfied that it may not belong to some allied 

 group. 



Locality and position. — Two hundred miles above the mouth of Milk 

 River; from the Fort Pierre group, or formation No. 4 of the Upper Mis- 

 souri Cretaceous series. 



Callista (Aphrodina!) t e n u i s , H. & M. 



Plate 5, figs. 1, a,b,c,d. 



Cytherea tenuis, Hall ami Meek (Mem. Am. Acad. Sci. and Arts, V (u. s.), 383, pi. 1, tig. 5. 

 Meretrix tenuis, Meek and Hayden (May, 1860), Proceed. Acad. Nat. Sci. Philud., XII, 185. 

 Dione ? tenuis, Meek (1864), Smithsonian Check-List N. Am. Cret. Fossils, 13. 



"Shell thin, ovate-orbicular; length and height nearly equal; beaks 

 elevated, nearly central ; anterior and posterior extremities rounded, the 

 latter somewhat broader; surface marked by concentric undulations, and fine 

 parallel striae. 



" Length, 0.45 inch ; height, 0.36 inch." 



The specimens from which the foregoing description. was made out, 

 being young or undeveloped individuals, and more or less distorted by pres- 

 sure, give an incorrect idea of the form of the adult shell. Small specimens 

 are generally more compressed and higher in proportion to length than the 

 larger, though the species varied somewhat in the relative proportions of 

 height and length, even at maturity. 



From a good series of specimens of various sizes, it is found that the 

 species may be characterized as follows : Shell thin, varying in form from 

 subcircular to transversely-ovate, rather gibbous at maturity. Anterior side 

 rather short, obliquely subtruncate above, and abruptly rounded below; base 

 forming a semi-ovate curve, being a little more prominent before than behind 

 the middle; posterior side narrowly rounded, or subtruncate; beaks moder- 

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

 and placed nearly half-way between the middle and (he anterior side ; lunule 

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



