PART I CHAZYAN AND RELATED BRACHIOPODS—COOPER OII 
nearly equal, strong convexity with the maximum curvature at about the middle 
and corresponding deep concavity. This species differs from O. matutina, another 
species from about the same horizon, in being larger, squarer, and deeper. It is 
larger and not so convex as O. dorsatiformis but is smaller and more convex 
than O. bellula. 
OPIKINA GREGARIA Cooper, new species 
Plate 238, B, figures 8-18 
Of about medium size for the genus, wider than long; hinge wider than or 
less than the midwidth; cardinal extremities varying from a slightly obtuse 
angle to acute and auriculate ; sides subparallel to oblique ; anterior margin some- 
what narrowly to broadly rounded; ornamentation fine and subdued, variable, 
that of the brachial valve somewhat finer than that of the pedicle valve, the cos- 
tellae numbering about 4 to the millimeter. Posterior margins with oblique 
wrinkles. 
Pedicle valve unevenly convex in lateral profile, the maximum convexity lo- 
cated just anterior to the middle; anterior profile broadly but moderately domed. 
Posterior two-thirds gently convex; anterior third bent strongly but broadly 
toward the brachial valve to form an angle of geniculation of 100° to 115°. An- 
terior slope rounded, short and steep; lateral slopes longer than the anterior one 
and less steep; broadly but gently swollen. Interarea short. 
Brachial valve fairly strongly concave in the median and posterior regions, 
most deeply concave just anterior to the middle; umbo concave; sides and an- 
terior margin moderately curved toward the brachial valve. Interior with mod- 
erate and beaded rim; cardinal process small; septa low and subdued. 
Measurements in mm.— 
Brachial Hinge 
Length length Midwidth width Thickness Height 
Holotype. </o<./s » sistas Sara's 18.0 17.2 21.4 20.3 3.5 6.2 
Paratype (117814a)..... 17.0 15.5 10.1 25.0 27 5.0 
GEIZ STAG) eye ses 17.3 16.9 22.1 25.4 22 5.7 
- (117814d)..... 13.9 13.2 16.4 16.0 1.9 By 
Types.—Holotype: 117814b; figured paratypes: 117814a,c-e; unfigured para- 
type: 117814f. 
Horizon and locality—Bromide formation (Mountain Lake member) in Okla- 
homa: On Oklahoma Highway 99, 3 miles south of Fittstown, Pontotoc County ; 
just west of the road on Russell Ranch, SE} sec. 3, T. 2 S., R. 3 E., Murray 
County. 
Discussion.—This species is characterized by moderate size, moderate con- 
vexity located anterior to the middle, fairly prominent ears, and unusually deli- 
cate ornamentation. In the latter respect it differs from O. formosa and 
O. expatiata, both Oklahoma species. It also differs from both of these in form 
and degree of geniculation. It differs from O. septata in having a more rounded 
geniculation and greater convexity. Opikina gregaria is shaped differently from 
O. transversa, less strongly geniculated and generally with less prominent ears. 
