PART! T CHAZYAN AND RELATED BRACHIOPODS—COOPER 453 
PLECTORTHIS SYMMETRICA Cooper, new species 
Plate 83, B, figures 16-21; plate 92, H, figures 45-50 
Shell of about medium size for the genus, wider than long and with the great- 
est width at about the middle. Hinge width about three-fourths the shell width. 
Cardinal extremities obtuse. Lateral margins somewhat narrowly rounded; an- 
terior margin broadly rounded. Surface costate; costae of pedicle valve slightly 
wider than those of brachial valve, with interspaces slightly narrower than the 
costae. Brachial costae narrower than those of the pedicle valve and the inter- 
spaces slightly wider than the costae. About 32 costae. Interspaces marked by 
occasional fine costellae and concentric fila. Surfaces of costae smooth. 
Pedicle valve moderately convex with the greatest convexity in the umbonal 
region ; anterior profile narrowly rounded medianly with moderate lateral slopes ; 
umbo gently convex; beak broadly obtuse and protruding slightly posterior to 
the posterior margin. Median region somewhat swollen; lateral and anterior 
slopes moderately steep. Posterolateral slope to cardinal extremities slightly 
concave and steeper than the slopes anterior to them. Interarea short, curved, 
apsacline. 
Brachial valve gently and evenly convex in lateral profile and with the great- 
est convexity at about the middle; anterior profile with a gentle, shallow median 
depression and slightly swollen flanks. Umbo sulcate; sulcus shallow, poorly 
defined and extending for about one-half to three-quarters the valve length. 
Flanks slightly swollen, posterolateral areas slightly flattened. Anterior half 
somewhat flattened ; interarea short, orthocline. 
Pedicle interior with short, narrowly divergent dental plates bounding a short 
muscle field. Brachial interior with widely divergent, flat, and broad brachio- 
phores braced by short, convergent brachiophore supporting plates. Cardinal 
process low but with a hebertelloid myophore. 
Measurements in mm.— 
Brachial Hinge 
Length length Width width Thickness 
EUONOEG Cede tae cake ee riet ce tees 9.2 8.6 11.4 8.8 4.7 
Paratype (110824b) ........... 8.4 8.2 II.0 8.8 4.8 
Types.—Holotype: 110824a; figured paratypes: 110828a,d; unfigured para- 
types: 110824b-f, 110828b,c. 
Horizon and locality—Bromide formation (Pooleville member-Oxoplecia 
gouldi zone) in Carter County, Okla.: At Rock Crossing of Hickory Creek, 
near center sec. 35; T. 5 S., R. 1 EINWF'sec? 26, T. 5 S., R. 1 E. 
Discussion—tThis species in its form suggests P. compacta, but it does not 
reach the large size and rotund form of that species. The valves of P. compacta 
are subequal in depth, whereas the brachial valve of P. symmetrica is shallow, 
the pedicle valve having the greater depth. Comparison with P. pennsylvanica 
is made under that species. 
