INVERTEBRATE PALEONTOLOGY. 187 



and Sciences must give an incorrect idea of the general form of this species, 

 since a majority of the specimens are much less broadly rounded posteriorly. 



Some varieties of this species, especially the broader forms, resemble 

 somewhat the species C. Owenana, but they are usually less concave in outline 

 just in advance of the beaks, and the pallial sinus is always different in form, 

 as may be seen by the figures. Some of the varieties also resemble our 

 C. Deweyi in form, as well as in the pallial sinus and muscular impres- 

 sions, but it is evidently a much thicker shell, and often distinctly more 

 gibbous. 



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 Mis- 

 souri Cretaceous. It was from this same locality and position that the speci- 

 men first figured by Professor Hall and the writer was obtained. 



Callista? pellucida, M &H. 



Plate 17, tigs. 10, a, b, c, d, c ; ami 12, a, b, c. 



Cytherea pellucida, Meek and Haydeu (1856), Proceed. Acad. Nat. Sci. Philad., VIII, 272. 



Meretrix pellucida, Meek aud Haydeu (May, 1860), ib., XII, 185. 



Vioue? pellucida, Meek (1864), Smithsonian Check-List N. Am. Cret. Fossils, VS. 



Shell extremely thin and fragile, broad-ovate, or subcircular, com- 

 pressed ; anterior side narrowly rounded; base forming a semi-oval curve; 

 posterior side rather broader than the other, rounded or subtruncated at the 

 extremity ; dorsum sloping gradually, with a slightly convex outline behind 

 the beaks, and concave and declining more abruptly in front; beaks moder- 

 ately elevated and placed a little in advance of the middle ; muscular 

 impressions very faintly marked ; pallial line quite distinct and provided with 

 a rather broad-triangular sinus extending almost to the middle of the valves. 

 Surface marked by fine lines of growth, which are epiite regular in the 

 umbonal region, but become more irregular near the free borders. 



Length (of a medium-sized, subovate variety), 0.8G inch ; height, 0.73 

 inch ; convexity, 0.41 inch. 



The most striking peculiarity of this species is its extreme thinness; the 

 substance of the shell being scarcely thicker than ordinary writing-paper. 

 This character and its more compressed form will readily distinguish it from 

 all the other species yet obtained from these formations. It varies consider- 

 ably in form, some individuals being longer than high, and others higher than 

 long. These varieties are quite strongly enough marked to constitute 



