[NVEKTEBKATE PAL2EONTOLOGY. ]77 



incurved nearly at right angles to the hinge-line, which is short and arched; 

 posterior umbonal slopes prominently rounded ; surface ornamented by fine 

 lines of growth and a few stronger concentric furrows, crossed on the posterior 

 half of the valves by about fifteen to twenty, very obscure, radiating costse, 

 that impart a distinctly crenate character to the free margins within, as well 

 as to some of the stronger marks of growth above the edge. 



Height, 0.G4 inch ; antero-posterior diameter, 0.59 inch ; convexity of 

 the two valves united, 0.50 inch. 



This species resembles the last two in the peculiarity of having its 

 radiating costse of the posterior side of the valves so faintly marked as almost 

 to escape attention, were it not for the fact that they cause the free margins 

 of that part of the valves, as well as some of the stronger marks of growth 

 above the margin, to be distinctly crenate. It may be at once distinguished, 

 however, from them both, by its much more elevated beaks, as well as by its 

 decidedly thinner and more ventricose valves. It agrees nearly in form and 

 surface-markings with the type of Pacliycardium, though it still differs remark- 

 ably in its extreme thinness. It is more closely related, however, in form, 

 as well as in having the entire posterior half of the valves occupied by the 

 radiating costae, to P. bisection, Forbes, from the Cretaceous rocks of Southern 

 .India. As that species, however, is described by Stoliczka as being moder- 

 ately thick, it must differ widely in that character from our species, which is 

 almost as thin as common writing-paper; it also has its basal margin much 

 less prominent behind, than that of P. bisection. 



Itocality and position. — Deer Creek, on the North Platte; in the Fox 

 Hills division of the Upper Missouri Cretaceous. Captain Raynolds's collec- 

 tion 



VENERIDjE. 



Genus CALLISTA, Poli. 



Si/uon.—CaUisia, Poli (1791), Test. Utr. Sic, I, 30.— H. and A. Adams (1857), Genera Recent Moll.. II, 424.— 



Meek and Hayden (1861), Proceed. Acad. Nat. Sci. Philad., 443.— Clienu (1862), Man. 



Conch., 86. — Stoliczka (1870), Paheout. Indica, III, 150 (as a subgenus of Cytherea). 

 Murdrix and Cytherea of authors; but not of Lam. (1799 and 1805), as restricted to such forms 



as V. meretrix, Linn. (= C. impudiea, Lam.). 

 Chione, Gray (1S36), Analist, * * ; and (1842), Synon. Brit. Mus., 74 and 95 (not Megerle von 



Muhlf., 1811). 

 Dioue, Gray (1847), Zoo). Proceed., IK!.— Gabb (1861), Synop. Mol. Cret., 120.— Conrad (1862), 



Proceed. Acad. Nat. Sci. Philad., 572 ; and (1865), Am. Journ. Couch., I, 6 ; and ib., 137.— 



Meek (1864), Smithsonian Check-List Cret. Foss. N. A., 1 .3 



23 ii 



