150 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I48 



STENOSCISMA VENUSTUM (Girty) 

 Plate 21, fig. 13 ; plate 22, figs. 1-10 ; plate 23, figs. 1-5 



Camarophoria venusta GIRTY, G. H., 1909, U.S. Geol. Survey Prof. Paper 58, 



p. 303, pi. 31, fig. 6-6c; KING, R. E., 1931, Univ. Texas Bull. 3042, p. 110, pi. 



34, figs. 29-31, pi. 35, figs. 1-5. 

 Stenoscisma venusta (Girty) COOPER, G. A., 1944, in Index foss. No. Amer., 



p. 315, pi. 120, figs. 38-40. 

 Stenoscisma venustum (Girty) COOPER, G.A., 1956a, Journ. Paleontology, vol. 



30, No. 3, p. 522, text fig. la. 



Adult shell large for genus; outline broadly subtrigonal, sides di- 

 verging between 65 and 115 degrees, averaging about 90, divergence 

 typically increasing with shell size ; profile moderately biconvex ; com- 

 missure uniplicate ; fold high, broad and only gently arched over crest, 

 beginning about 8-15 mm. anterior to brachial beak; sulcus rather 

 shallow except at anterior, beginning 12-18 mm. anterior to pedicle 

 beak. Costae moderately strong, blunt crested, beginning 1-5 mm. ante- 

 rior to beaks, numbering five to eight, normally five or six on fold, one 

 less in sulcus, four to six on flanks, becoming lower and weaker later- 

 ally, rarely one or two on fold or sulcus bifurcating anteriorly ; stolid- 

 ium broad, fanlike, finely concentrically ornamented, beginning just 

 behind widest part of shell, not continuous from flanks to fold, better 

 developed on brachial valve, may be reduced or absent; growth lines 

 fine and closely spaced, with a few stronger laminae at irregular inter- 

 vals. 



Pedicle valve gently convex transversely and from beak to flanks, 

 more strongly convex through sulcus ; umbonal region relatively low, 

 not swollen ; beak about normal length for genus, suberect to erect but 

 not strongly hooked ; beak ridges short, blunt ; delthyrium triangular, 

 small, constricted along sides and at anterior by conjunct deltidial 

 plates, leaving narrow, slitlike foramen normally opening directly dor- 

 sally ; posterolateral flanges elongate, narrow, normally covered by 

 overlapping edge of opposite valve. 



Brachial valve strongly convex transversely, moderately strongly 

 convex along crest of fold, convexity rather uniform, valve nowhere 

 strongly swollen or flattened ; beak bluntly pointed, apex within pedicle 

 valve, hidden by deltidial plates. 



Pedicle valve interior with blunt hinge teeth elongate parallel to 

 sides of valve ; dental plates continuous with hinge teeth, slightly con- 

 vergent toward floor of valve, more abruptly curved immediately above 

 floor, joining one another to form boat-shaped spondylium, supported 

 by low median septum ; space between floor of valve and outside of 

 spondylium may be filled with callus. Relatively deep, transverse 



