PART I CHAZYAN AND RELATED BRACHIOPODS—COOPER 301 
ORTHAMBONITES DINORTHOIDES Cooper, new species 
Plate 33, E, figures 20-27 
Shell of moderate size, wider than long, with the length equal to about two- 
thirds the width. Hinge slightly narrower than the greatest shell width which 
is located near the middle. Cardinal extremities obtusely angular. Lateral mar- 
gins gently rounded; anterior margin broadly rounded. Anterior commissure 
slightly sulcate. Surface costate with the number of costae varying from 21 to 
25. Costae narrowly rounded and with interspaces wider than the ribs. Inter- 
spaces marked by fine radial and concentric lines. 
Pedicle valve very gently convex in lateral profile with greatest convexity lo- 
cated at the umbo; anterior profile moderately convex. Beak well defined, 
projecting a little posterior to the posterior margin, obtuse, umbo narrowly 
swollen. An ill-defined fold formed at the umbo by the central 3 costae. Fold 
best defined in the posterior half but flattened at the front where there may 
be a shallow depression in its place. 
Brachial valve gently convex in lateral profile ; gently convex in anterior profile 
and with the center sulcate. Sulcus narrow and shallow and containing 2 costae. 
Sulcus originating at the umbo and extending to the front margin. Flanks of 
shell gently convex bounding the sulcus. Slopes to cardinal extremities gentle ; 
posterolateral extremities flattened. 
Interiors —Muscle area of pedicle valve more elongate than usual in the genus 
and with a truncate anterior. Diductor scars elongate but adjustor scars small 
and inconspicuous. Cardinal process with short shaft and narrow myophore. 
Measurements in mm.— 
Hinge 
Length Width width Thickness 
Holotype (pedicle valve) ............ 13.2 17.5 16.8 2.5? 
Paratype (brachial valve 1og821a) ... 15.6 21.0 15.0? 1:75? 
Types.—Holotype: 109823b; figured paratypes: 109821a,d, 109822a,b, 
109823a,c,d; unfigured paratypes: 109821b,c,e,f, 109822c-h, 109823e-h. 
Horizon and locality.—Oil Creek formation in Oklahoma: Sec. 14, T. 2 S., R. 
1 E., about 5 miles north-northwest of Springer, Murray County; 3 miles east 
of Pooleville, Murray County ; 614 feet above the base on west side U. S. High- 
way 77, NELNE3 sec. 25, T. 2 S., R. 1 E., vicinity of Springer, Carter County. 
Discussion.—As the name of this species implies, it is quite similar to the genus 
Dinorthis. The dinorthoid features are in the interior rather than on the exterior. 
The elongate diductor scars which flare a little laterally at their extremities, and 
the truncate front of the muscle field are most suggestive of Dinorthis. Similari- 
ties to this genus also occur in the brachial valve. The cardinal process with a 
well-defined myophore suggests Dinorthis, but its form and shaft are more like 
that of Orthambonites. Although the muscle field, like the cardinal process, sug- 
gests Dinorthis, the small development of the adjustor scars and fine radial orna- 
ment suggest Orthambonites rather than Dinorthis. 
Orthambonites dinorthoides reminds one of O. paucicostata Ulrich and Cooper 
