242 



Dorsal valve circular in outline except where truncated by the hinge- 

 line; regularly convex; beak less prominent than that of the other valve, 

 extending beyond the hinge-line; area very narrow. 



Surface marked l)y very numerous almost indistinct radiating costje 

 and l)y somewhat strong concentric markings. 



Position and Locality.— Upper Red Wall Group, north bank of White 

 River Canyon, tAvelve miles southwest of Fort Apache, Arizona. 



DIELUSMA King (1859). 



DIELUSMA BOUVIDINES (Morton). 

 Plae, Fig. 11. 



White's description (in part)*: Shell ovate or elongate-ovate in outline: 

 sides behind the middle laterally compressed. Ventral valve strongly 

 arcuate from front to lieak. the curvature lieing greatest behind the 

 middle, rather more capacious than the other valve; beak prominent, in- 

 curved; foramen moderately, not squarely, truncating the beak, but open- 

 ing obliquely backward, mesial sinus lu'oad. and more or less distinct 

 at the anterior part of the valve, but becoming obsolete at or behind the 

 middle. Dorsal valve generally almost straight along the median line 

 from front margin to a little behind the middle, from which part it gently 

 curves to the beak; gently and somewhat unifoiiuly convex from side to 

 side, without a mesial fold. 



Surface nearly smooth; shell structure finely punctate. 



Position and Locality.— Upper Red Wall Group, Fort Apache. Arizona. 



SEMINULA McCoy (1844). 



SEMINULA ARGENTIA Shepard. 



Plate, Figs. 12 a, b, c, p. 



Shell varying considerable in outline, generally subovate; seldom as 



wide as long, usually moderately gibbous. Init sometimes old shells are 



much inflated. Ventral valve generally a little more capacious than the 



dorsal; beak rather prominent, incurved; mesial sinus usually not very 



deep, and becoming obsolete about the middle. 



*U.S. Geog. Surv. W. of the 100th meridian, Vol. IV, P. 141, PI. XI, Figs. lOa.b.c. 



