PART I CHAZYAN AND RELATED BRACHIOPODS—COOPER 799 
vexity located at about the middle ; anterior profile strongly convex in the middle 
with long and moderately steep and somewhat concave lateral slopes. Beak 
small, umbo somewhat swollen, the swelling continued forward as a poorly de- 
fined fold; flanks gently swollen and descending fairly steeply to the lateral 
slopes; anterior slope long and steep. 
Brachial valve deeply concave with the deepest part forming a poorly defined 
sulcus extending from umbo to anterior margin ; areas bounding sulcus less con- 
cave than the median region; subperipheral regions concave. Interior with 
slightly divergent median septa elevated near the middle; subperipheral rim 
moderately thickened ; anterior slope moderately thickened, pustulose, and steep. 
Measurements in mm.— 
Brachial Hinge : 
Length length Midwidth width Thickness Height 
1 = (0) (5 347) as yy | 7p 12.2 13.4 1.6 a7 
Paratype (117536a).. 6.1 5.8 9.6 12.0 1.2 2.0 
4 CLIZ5S7)). LLM FE 12.1 14.2? 1.5 2.6 
Types.—Holotype: 117536b; figured paratypes: 117535b,c, 117536a, 117538a, 
117539a,b ; measured paratype: 117537. 
Horizon and locality—Bromide formation (Pooleville member-O-roplecia 
gouldi zone) in Oklahoma: At Rock Crossing of Hickory Creek near center 
Sec, 95,.1. 5 o- WoL L.,.Criner Fills, Carter County: Spring: Creek, ‘Sec. “17, 
T.28S., R. 1 W., Murray County ; (370 to 390 feet above base), creek bed west 
of U. S. Highway 77, SEANE$ sec. 25, T. 2 S., R. 1 E., Carter County. 
Discussion —This species is a thin-shelled form characterized by considerable 
depth to the brachial valve, fine, beaded costellae, alate cardinal extremities, 
moderate convexity, and well-differentiated costellae. This species differs from 
S. vulgata, another Oklahoma species with which it might be confused, in the 
differentiated costellae, those of S. vulgata not being so well grouped as those of 
S. variabilis. This feature of the costellae distinguishes S. variabilis from simi- 
lar appearing Appalachian species such as S. compacta and S. socialis. Sower- 
byella lebanonensis has differentiated costellae, but its outline and profiles are 
different from those of the Oklahoma species. Sowerbyella perplexa has more 
regularly differentiated costellae than S. variabilis and has differently shaped 
valves. 
SOWERBYELLA VARICOSTELLATA Cooper, new species 
Plate 196, A, figures 1-8 
Shell small, width at hinge not quite twice the length; anterior margin broadly 
rounded; lateral margins gently convex. Cardinal extremities acute, subalate. 
Posterior margin nearly straight. Surface marked by costellae of unequal size 
that appear in 4 generations. On the pedicle valve the median costella of the 
first generation is the strongest one and has the greatest size and height. On 
each side of this costella the flanks are marked by 2 to 3 first generation costellae 
that extend from the beak to the front margin. Within the areas marked off 
by these major costellae 3 generations of costellae may be counted. The second 
