182 UNITED STATES GEOLOGICAL SURVEY OF THE TERRITORIES. 



shells may yet be found to fall into sections of this group. Aphrodina and 

 Vqsiniopsis certainly occur in the Cretaceous; the latter also passing into 

 the Lower Eocene, but not above, so far as yet known. Callista proper, or 

 species that seem scarcely, if at all, separable from this section, also occur in 

 the Cretaceous, and a few elongated forms resembling Macrocallista have 

 been found in rocks of that age ; but I am not quit e sure that the hinges of 

 these shells are known to agree with the typical species of the group. Both 

 of the latter two sections, however, as well as Dione and Pilar, are abun- 

 dantly represented through the Tertiary, and by numerous living species in 

 our existing seas, where the genus appears to attain its greatest numerical 

 development. 



Callista ( Dosi nio psis ? ) Deweyi, M. & H. 



Plate 17, figs. 15, a, b, c, <?, e. 



Cytherea Deweyi, Meek and Hayden (1856), Proceed. Acad. Nat. Sci. Philad., VIII, 83. 



Meretrix Deweyi, Meek and Hayden (18G0), ib., XII, 185. 



Callista Deweyi,Meek and Hayden (1861), ib., XIII, 143. 



Dione Dewey i, Meek (1804), Smithsonian Check-List N. Am. Cret. Fossils, 13. ■ 



Shell subcircular, or very broad-suboval, rather thin, moderately convex ; 

 lateral margins rounded, the posterior side being a little broader than the 

 other; dorsum sloping gradually with a slightly convex outline behind the 

 beaks, and concave and more abrupt in front ; base semi-oval ; escutcheon 

 lanceolate ; beaks not very prominent, somewhat gibbous, incurved, nearly 

 touching, and placed a little in advance of the middle ; muscular impressions 

 shallow, anterior one narrow-oval, posterior one broad-ovate ; sinus of the 

 pallia! impression broad, triangular, its sides converging at an angle of about 

 35°, extending obliquely forward and upward nearly to the middle of the 

 valves, very slightly obtuse at the immediate extremity. Surface marked by 

 fine, regular, prominent lines of growth. 



Length, 0.96 inch ; height, 0.83 inch ; gibbosity, 0.55 inch. 



This is a very neat, symmetrical shell, which appears to vary somewhat 

 in form at different localities. At any rate, specimens that seem not distinct 

 from it, obtained on the Yellowstone, are often in outline a little more oval 

 transversely ; their length being slightly greater in proportion to their height, 

 As these differ in no other respect, so far as has yet been determined, and 

 there are intermediate gradations of form, I do not think it probable that 

 they belong to different species. ■ 



