278 REPORT UNITED STATES GEOLOGICAL SURVEY. 



very clearly defined species, tbe numerous examples of it in the collec- 

 tions showing it to have been subject to little essential variation. It 

 belongs to the typical section of the subgenus, and not to the section of 

 long falciform species represented by A. carinata Lamarck and A. ma- 

 croptera d'Orbigny. 



Position and locality. — Fox Hills Group ; valley of Weber Eiver, near 

 Coalville, Utah. 



Genus EXOGYEA Say. 



EXOGYRA VALKERI (sp. UOV.). 

 Plate 1, figs. 1 a and h. 



Shell large, broadly subelliptical or oblong in marginal outline, de- 

 pressed ; test moderately thick ; larger valve depressed, irregularly con- 

 vex ; dorsal portion flattened and almost at right angles with the plane 

 of the shell, especially near the umbo, from which portion to the general 

 surface it is abruptly rounded ; umbo incurved, making more than one 

 complete volution, much depressed beneath the general exterior surface 

 of the valv'e ; muscular impression large, subovate in outline, subcentral; 

 ligainental groove comparatively small, moderately deep, close to the 

 outer margin ; pallial margin having a comparatively broad space along 

 its whole length marked by faint radiating crenulations. Smaller valve 

 nearly flat, or slightly and more or less irregularly concave ; its umbo 

 flattened and equally incurved with that of the other valve. 



Surface of the larger valve marked by the ordinary lines of growth, 

 and, especially upon the incurved side, by irregular knotted ridges upon 

 the concentric lines. Surface of the smaller valve marked by very 

 numerous and distinct, coarse, concentric laminae of growth. 



Length, 14 centimeters ; width, 10£ centimeters; thickness, 5 cen- 

 timeters. 



This species belongs to the section of the genus Exogyra, which is rep- 

 resented by such species as E. plicata Chemnitz (sp.), E. boussingaultii 

 d'Orbigny (sp.), and E. texana Eoeiner; the last-named species being 

 also from the Cretaceous rocks of Texas. It differs, however, from all three 

 of these species in being destitute of all radiating plications or folds ex- 

 cept those of concentric increment, as well as in other less conspicuous 

 but still obvious characteristics. 



Position and locality. — Cretaceous strata ; Salado, Bell County, Texas, 

 where it was collected by Mr. D. H. Walker, and in whose honor the 

 specific name is given. 



Genus PLACITNOPSIS Morris & Lycett. 

 Placunopsis hilliardensis (sp. nov.). 



Plate 7, fig. 14 a. 



Shell small, broadly oval or subcircular, slightly oblique ; test thin, 

 fragile, papyraceous; margins somewhat irregular; upper valve mod- 

 erately convex ; umbo submarginal, the apex depressed and not clearly 

 defined. Surface conspicuously marked with numerous coarse, radiating, 

 irregularly undulating, abruptly raised lines, which are wider than the 

 spaces between them, and some of which appear to have ended at the 

 border, or upon imbricating concentric lines, as tubular or semicylin- 

 drical processes. Diameter of the few examples obtained, about 12 milli- 

 meters. 



