staxton.] SPONDYLIDJE. 69 



cylindrical processes. Diameter of the few examples obtained, about 

 12 ul,u . 



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

 seems to be a species of true Placunopxis Morris & Lycett, and to be 

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

 M. & L., Monog. Gr. Ool. Mol., p. 6, PI. 6, Figs. 8, 8a, and 86) is oi 

 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 1 suggests that the Anomia subtrigonalis of Meek 

 and Hayden from the Fort Pierre group of the Upper Missouri prob- 

 ably belongs to the genus Placunopsis. Our species differ conspic- 

 uously from that one in being radiately marked, and also in the charac- 

 ter 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 

 indicate the genus to which it is referred. In its marginal outline and 

 surface markings it resembles the Gapulus occidentalis of Hall and 

 Meek, from the Upper Missouri river region, as figured by those 

 authors, but the distinct laminated pearly texture of the shell sub- 

 stance would forbid its reference to that genus if its other characters 

 were less doubtful. 



"Position and locality, — Strata of the Fox Hills group, near Hilliard 

 station, Union Pacific railroad, Wyoming. 77 * 



Additional collections from the neighborhood of the original locality 

 have added nothing to our knowledge of the generic relationship of 

 this species. It seems to me that shells of this type should be referred 

 to Anomia, at least until their internal structure is known. When 

 this ! description was written all of the Cretaceous sandstones in the 

 neighborhood of Hilliard, which is on Sulphur creek not far from Bear 

 river, were referred to the "Fox Hills" or Montana formation. It is 

 now known that a large part of these sandstones belong to the Colo- 

 rado, but it has not been definitely determined whether the uppermost 

 beds containing this species, Neritina incompta and a few other forms, 

 should be assigned to it or to the Montana. 



SPONDYLID^E. 



GENUS PLICATULA LAMARCK. 



Plicatula hydrotheca White. 



PI. ix, Figs 1 and 2. 



riicatula hjdrotheca White, 1876, Geol. Uinta. Mts. p. 113; 1879, 11th Ann. Rcpt. U. 

 S. Geol. Snr. Terr., p. 279, PI. 6, Figs. 3a and b. 



Revised description : 



" Shell of ordinary size, a little, obliquely and* irregularly subovate 

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



»U. S. Geol. Surv. Terr., vol. ix, 4to., p. 23. 



