white.] PALEONTOLOGY — CEETACEOUS FOSSILS. 279 



Although the hinge and interior of this shell are not known, it seems 

 to be a species of true Placunopsis Morris & Lycett, and to be nearly- 

 related to their typical species, although the latter (P. jurensis M. 

 & L., Monog. Gr. 6ol. Mol. p. 6, pi. 6, figs. 8, 8 a, and 8 b) is of Jurassic 

 age, while the former is Cretaceous. This appears to be the only known 

 species of this genus in the Cretaceous rocks of the United States, but 

 Mr. Meek (U. S. Geol. Suit. Terr. vol. ix, 4to, p. 23) suggests that the 

 Anomia subtrigonalis of Meek and Hayden from the Fort Pierre Group 

 of the Upper Missouri probably belongs to the genus Placunopsis. Our 

 species differs conspicuously from that one in being radiately marked, 

 and also in the character of its marginal outline and general aspect. 



It is true that neither the under valve nor the interior of our species is 

 known, but the characters, so far as they are known, very plainly indi- 

 cate the genus to which it is referred. In its marginal outline and sur- 

 face-markings it resembles the Capidus occidentalis of Hall and Meek, 

 from the Upper Missouri River Eegion, as figured by those authors, but 

 the distinct laminated, pearly texture of the shell substance would forbid 

 its reference to that genus if its other characters were less doubtful. 



Position and locality. — Strata of the Fox Hills Group j near Hilliard 

 Station, Union Pacific Railroad, Wyoming. 



Genus PLICATULA Lamarck. 



Plicatula hydeotheca White. 



Plate 6, fig. 3 a and i. 

 Plicatula Tiydroilieca White, 1876, Powell's Eep. Geol. Uinta Mts. p. 113. 



Shell of ordinary size, a little obliquely and irregularly subovate in 

 marginal outline; rostral region narrowed, its sides nearly straight or 

 only slightly convex ; the remainder of the free border somewhat regu- 

 larly convex or rounded ; lower valve broadly convex ; hinge-teeth well 

 developed ; hgamental fosset moderately large ; upper valve nearly flat 

 or slightly concave in the rostral region. Surface of both valves marked 

 by small, slightly raised, radiating pbcations, which are crenulated, a 

 little irregular, increase in number both by bifurcation and implantation, 

 and are more or less distinct upon all parts of the surface of both valves. 



Length, 3 centimeters ; greatest breadth, 24 millimeters. 



Position and locality. — Cretaceous strata, probably equivalent with the 

 lower portion of the Colorado Group ; head of Waterpocket Canon, 

 Southern Utah. 



CoUected by Mr. G. K. Gilbert. 



Genus PTEEIA ScopoH. 

 Pteeia paekensis White. 



Plate 3, fig. 3 a. 



Avicula parlensis White, 1876, Powell's Eep. Geol. Uinta Mts. p. 115. 



Shell rather small, slightly inequivalve, very oblique, elongate, thin at 

 all the margins except the cardinal ; anterior wing of ordinary size and 

 shape ; posterior wing rather large and long ; both valves broadly and 

 somewhat regularly convex ; greatest breadth of the shell behind the 

 middle ; antero-basal border broadly convex ; posterior extremity reguv 



