442 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 127 
Greatest shell width near the middle. Anterior commissure rectimarginate. 
Valves covered by rounded costellae as wide or wider than the interspaces, about 
7 to II costellae in 5 mm. at the front margin. 
Pedicle valve in lateral profile gently convex. Median fold slightly developed 
in the posterior half; umbonal region hemipyramidal, flattened in profile; slopes 
to cardinal extremities moderate, front half flattened to slightly concave. Inter- 
area long, strongly apsacline. 
Brachial valve strongly convex in lateral and anterior profiles with the greatest 
convexity at the middle. Sulcus shallow, originating at the beak and extending 
to the front margin or lost just before reaching the margin. Median region of 
valve strongly swollen in adults with steep slopes in all directions from the mid- 
dle. Interarea moderately long, curved, apsacline. 
Measurements in mm.— 
Brachial Hinge 
Length length Width width Thickness 
Holotype. 1a. teal. rede 18.7 18.8 23.2 21.2 8.9 
Paratype (110561a) ......... 18.6 18.7 24.2 21.6 9.3 
Types.—Holotype: 110562a; figured paratypes: 110561a, 117991a; unfigured 
paratypes: 110561b-d, 110562b, 117991b,c. 
Horizon and locality —Bromide formation (Mountain Lake member) in Okla- 
homa: Sec. 25, T.2S., R. 1 E., 4 mile west of U. S. Highway 77; 3 miles north- 
northeast of Springer, Carter County. Spring Creek, sec. 17. T. 2 S., R. 1 W., 
Murray County ; NW3 sec. 27, T. 1 S., R. 1 W., midway between Davis and 
Dougherty, West Spring Creek, east of Pooleville, Murray County. 
Discussion.—This species in its nearly biconvex valves strongly suggests the 
closely similar genus Multicostella. Unlike that genus the pedicle valve is de- 
pressed at the anterior half, and the fold and sulcus are short and obscure. The 
species is finely costellate and differs from C. lebanonensis, which it approaches 
in size, by its more convex valves. It is smaller than the other described Okla- 
homa species except C. crassa to which it has no resemblance. 
CHAULISTOMELLA RECTANGULATA Cooper, new species 
Plate 69, B, figures 7-16 
Shell of about medium size for the genus, rectangular in outline; wider than 
long with the hinge forming the widest part or slightly narrower than width at 
middle. Sides sloping obliquely in a median direction; anterior margin broadly 
rounded ; anterior commissure rectimarginate to broadly uniplicate; surface mul- 
ticostellate, costellae narrowly rounded, closely crowded, about 3 in I mm. at 
the front margin. 
Pedicle valve variable in lateral profile, posterior half flattened to gently con- 
vex; anterior half depressed convex to flat; median fold indistinct and narrow, 
fairly well defined on the umbo, indistinct in the anterior third; median region 
flattened ; posterolateral extremities deflected slightly toward the brachial valve ; 
interarea long, steeply apsacline to nearly catacline. 
