PART I CHAZYAN AND RELATED BRACHIOPODS—COOPER 791 
SOWERBYELLA PLICATIFERA Cooper, new species 
Plate 203, A, figures 1-15 
Shell of about the usual size for the genus, wider than long and with the width 
at the hinge nearly twice the length. Cardinal extremities acute to mucronate. 
Lateral margins oblique and straight to oblique and somewhat rounded. An- 
terior margin broadly rounded to nasute. Anterior commissure often wrinkled 
by 3 more or less strong plications, 1 on the pedicle valve and 2 on the brachial 
valve. Surface multicostellate ; costellae crowded, narrowly rounded, with inter- 
spaces narrower to slightly wider than the width of the costellae, varying from 
nearly equal in size to well differentiated in size with as many as 3 fine costellae 
separating the larger ones, about 120 to 160 in number. At the front margin 5 
to 7 costellae occupy a space of 1 mm. Interspaces occupied by fine, elevated con- 
centric fila. Costellae ornamented by small crescent-shaped spines, about 6 to 
the millimeter, giving them a beaded effect. 
Pedicle valve forming a low arch in anterior profile with a narrowly rounded 
crest and sides sloping gently to the margins. In lateral profile the curvature is 
moderate and the greatest curvature is slightly posterior to the middle. Beak 
inconspicuous; umbo narrowly elevated and extended forward to the front 
margin as a low narrow fold outlined by a shallow depression on each side. 
Flanks narrowly but often obscurely folded by a low plica that meets the front 
margin at the anterolateral extremities. Slopes to cardinal extremities concave 
but gentle. Interarea short, slightly curved and apsacline in position. 
Brachial valve with most concave portion near the middle and with the con- 
cavity extending to the front margin as a narrow sulcus. Sulcus bounded by 
low, narrowly rounded plications that extend to about the middle of the valve. 
Outside each plica a shallow sulcus may occur, but often, especially in younger 
specimens, the shell is gently concave to the cardinal extremities where it is 
somewhat flattened. Cardinal extremities slightly deflected toward the pedicle 
valve. Interarea short, hypercline. 
Interior: Not known beyond the fact that the brachial valve possesses typical 
Sowerbyella septa. 
Measurements in mm.— 
Brachial Hinge 
Length length Width width Thickness Height 
Holotype). 2202.02.02 8.90 8.2 13.6 15.0 1.8 28 
Paratype (117512a).. 8.6 8.0 12.5 12.9 1.4 2.6 
4 (iP7S128) 24178 7.4 12.0 14.5 1.9 2.6 
“s (11751Ib).. 7.8 7.6 11.9 13.8 ie Ze 
Types.—Holotype: 117512b; figured paratypes: 117511a, 117512a; unfigured 
paratypes: 117510, I17511b,c. 
Horizon and locality—Bromide formation (Mountain Lake member) in Okla- 
homa: West of Nebo, Murray County; NW} sec. 25, T. 2 S., R. 1 E., + mile 
west of U. S. Highway 77, Carter County. 
Discussion.—This species is characterized by strongly beaded costellae, deep 
brachial valve, acute cardinal extremities, and the presence of fairly strong folds, 
