PART I CHAZYAN AND RELATED BRACHIOPODS—COOPER 351 
ridge; cardinal process a thin septum; brachiophores stout; chilidium slightly 
developed. 
Measurements in mm.—Holotype, length 8.2, brachial length 7.8, width 10.0, 
hinge width 11.5, thickness 5.1. 
Types.—Holotype: 1100044 ; figured paratype: 110004b. 
Horizon and locality—Bromide formation (Mountain Lake member-middle 
zone 7 of Decker) in Oklahoma: Rock Crossing of Hickory Creek approxi- 
mately center sec. 35, T. 5 S., R. 1 E., Criner Hills, Carter County. 
Discussion—This species is readily distinguished from H. sulcata which is 
much larger, very differently shaped, and differently ornamented. 
HESPERORTHIS ? DECIPIENS (Phleger) 
Orthis decipiens PuueceEr, Bull. S. California Acad. Sci., vol. 32, pt. 1, p. 17, pl. 1, fig. 2, 1933. 
Horizon and locality—Barrel Spring formation in California: In Barrel 
Spring Canyon, Inyo Mountains. 
HESPERORTHIS DUBIA Cooper, new species 
Plate 51, B, figures 7-13 
Shell fairly large for the genus, slightly wider than long with the hinge form- 
ing the widest part ; cardinal extremities slightly auriculate; sides gently rounded ; 
anterior margin narrowly rounded ; anterior commissure rectimarginate ; surface 
marked by about 34 elevated, narrowly rounded costae slightly wider than the 
separating grooves ; 2 or 3 fine costellae occupying the striae. 
Pedicle valve very gently but unevenly convex in lateral profile, the greatest 
convexity in the posterior third or half; anterior half somewhat flattened ; an- 
terior profile narrowly humped in the median region with long, flat, and mod- 
erately steep slopes to the margins. Anterior slopes long and gently convex; 
beak obtuse, forming an angle of 140° to 150°; umbo broadly swollen; median 
region gently swollen ; interarea very slightly curved ; strongly apsacline ; pseudo- 
deltidium absent. 
Brachial valve almost flat in lateral profile but with anterior portion slightly 
concave ; anterior profile nearly flat; umbonal region gently swollen; median re- 
gion and most of anterior depressed into a broad and shallow sulcus the anterior 
part of which is slightly elevated near the margin; flanks bounding sulcus gently 
and somewhat narrowly swollen; posterolateral areas flattened and deflected 
slightly toward the pedicle valve; interarea short. 
Measurements in mm.—Holotype, length 15.4, brachial length 13.7, width 17.9, 
hinge width 17.3, thickness 6.4. 
Type.—Holotype: 116919. 
Horizon and locality—Lower Sevier formation in Tennessee: + mile north- 
east of Miser, Louisville (T.V.A. 138-SE) Quadrangle. 
Discussion—This is a large and rare species approaching H. tricenaria in 
size. It differs from that species in the lesser convexity of the pedicle valve, 
