PART I CHAZYAN AND RELATED BRACHIOPODS—COOPER 305 
Pedicle valve gently convex in lateral profile; greatest convexity located in 
the posterior half; anterior profile broadly convex, somewhat sharply pointed 
in the middle but with long, gently descending slopes; umbo gently swollen; 
anterior to umbo a narrow fold consisting of 5 costae extends to the anterior 
margin; flanks bounding fold gently swollen. 
Brachial valve very gently convex in lateral profile with the greatest con- 
vexity in the posterior half; anterior profile broadly convex and indented medi- 
ally ; sulcus originating at the umbo and extending to the anterior margin where 
it occupies about one-third the width. Flanks bounding sulcus somewhat swol- 
len; posterolateral extremities flattened; umbonal slopes short and gentle. 
Measurements in mm.—Holotype, length 4.5, brachial length 4.1, width 5.5, 
hinge width 4.7, thickness 2.2. 
Type.—Holotype: 116882. 
Horizon and locality.—Tulip Creek formation (about go feet below top) in 
Oklahoma: On east edge of sec. 25, T.2 S., R. 1 E., east side of U. S. Highway 
77, 24 miles north of Springer, Carter County. 
Discussion.—This species is similar to O. minutus in size but differs in having 
more distantly spaced and fewer costae, a deeper sulcus and different proportions. 
ORTHAMBONITES MINUSCULUS (Phleger) 
Orthis minusculus PHuEGER, S. California Acad. Sci., Bull., vol. 32, pt. 1, p. 7, pl. 2, figs. 6, 7 
(reprint), 1933. 
Orthis minuscula Phleger, ULricH and Cooper, Geol. Soc. Amer. Special Pap. 13, p. 101, 
pl. 15D, figs. 22-25, 1938. 
Types.—Hypotypes : 91310a-d. 
Horizon and locality—Mazourka formation, Inyo Mountains, Calif. Upper 
Pogonip group (sponge beds), Ikes Canyon, Toquima Range, Roberts Moun- 
tains (1°) Quadrangle, Nevada. 
ORTHAMBONITES MINUTUS Cooper, new species 
Plate 42, A, figures 1-9 
Shell minute, wider than long with hinge about equal to the midwidth or 
slightly wider or narrower; sides gently rounded; anterior margin broadly 
rounded; surface marked by upward of 40 closely crowded, narrowly rounded 
costae. 
Pedicle valve gently convex in lateral profile with the greatest convexity at 
about the middle; anterior profile broadly convex and with long lateral slopes. 
Median region from umbo to anterior margin somewhat swollen to form an in- 
distinct fold; interarea moderately long, strongly apsacline; teeth large; dental 
plates small, receding, and bounding a broad and fairly deep delthyrial cavity. 
Individual muscle scars not preserved. 
Brachial valve with a gently convex lateral profile and a lobate anterior pro- 
file; sulcus originating on the umbo, widening to the anterior margin where it 
occupies nearly half the shell width; flanks bounding sulcus moderately swollen ; 
