PART I CHAZYAN AND RELATED BRACHIOPODS—COOPER 313 
Measurements in mm.— 
ui 
Length Width width Thickness 
Holotype (pedicle valve) ............ 13.5 16.4 12.5 3.5 
Paratype (brachial valve 109844b).... 13.0 15.5? 13.4 ? 
Types.—Holotype: 1098442 ; figured paratype: 109844b; unfigured paratypes: 
109844c-e. 
Horizon and locality—Oil Creek formation in Oklahoma (75 to 100 feet below 
the top): On West Spring Creek, SW3 sec. 30, T. 1 S., R. 1 W., east of Poole- 
ville, Murray County. 
Discussion.—This species is most suggestive of Orthis michaelis that occurs 
in the Swan Peak formation of Utah. The Oklahoma species differs in the lesser 
convexity of both valves, the more inclined pedicle beak, the narrower pedicle 
muscle field and the more ponderous cardinalia and median ridge. 
O. subconvexus has some of the features of O. acutiplicatus and O. dinor- 
thoides. It differs from both in having more closely spaced costae and from 
O. dinorthoides in not having as elongate or such flaring pedicle muscles. 
ORTHAMBONITES TENNESSEENSIS Cooper, new species 
Plate 49, D, figures 16-21; plate 82, D, figures 13-20; plate 82,1, figure 4o 
Shell subquadrate in outline, hinge slightly narrower than the greatest shell 
width which is at the middle; sides gently curved; anterior margin narrowly 
rounded ; biconvex, the pedicle valve having the greater depth. Surface marked 
by 18 strong, subangular costae. 
Pedicle valve fairly strongly convex in lateral profile and with the greatest 
convexity in the median region; anterior profile somewhat narrowly convex; 
umbo narrow and protruding ; median region swollen and with a somewhat in- 
distinct narrow fold extending to the anterior margin; lateral slopes gently con- 
cave and fairly steep in the posterolateral region. Interarea narrowly curved, 
apsacline. Interior with short, thick dental plates; umbonal cavities filled; teeth 
small but with deep fossettes. Lateral plates slightly developed ; diductor scars 
protruding slightly anterior to the anterior end of the delthyrial cavity ; small but 
thick ridge extending anterior to the muscle field. 
Brachial valve moderately and evenly convex in lateral profile and with maxi- 
mum convexity at about the middle; umbonal region narrowly curved; anterior 
profile broadly convex and indented medially ; sulcus narrow and shallow on the 
umbo, widening anteriorly, and occupied by 2 costae. Flanks bounding sulcus 
gently swollen; posterolateral slopes short and steep. Interior with thickened 
notothyrial platform, simple cardinal process and moderately long brachiophores ; 
median ridge low, extending to the middle. 
Measurements in mm.— 
Brachial Hinge 
Length length Width width Thickness 
BOGE UE sic inrsis «Act a» «00 sitter + ein .aiarals 10.9 0.7 10.9 9.2 6.4 
Paratype (pedicle valve 116878a)... 9.1 ? 10.7 9.8 3.4 
ii (brachial valve 116878c).. ? 8.7 10.8 9.2 2.2 
