PART I CHAZYAN AND RELATED BRACHIOPODS—COOPER 739 
shallow sulcus occupied by about 16 costellae. Lateral areas bounding sulcus 
slightly convex, depressed below the median fold. Interarea long, apsacline. 
Brachial valve with a shallow, narrow sulcus originating at the beak and ex- 
tending distinctly for slightly more than half the length where it becomes 
shallower and passes into a broad gentle fold. Lateral areas convex except near 
the posterior margin where they are slightly sulcate. Anterolateral slopes mod- 
erately steep. Interarea moderately long, hypercline. 
Interiors of both valves greatly thickened and all structures enlarged. Del- 
thyrial cavity partially filled by excess shell; subperipheral callosities often pon- 
derous. Inside the brachial valve the median septa are welded into a single 
spoonlike process highest at the center. Internal surfaces strongly granulose. 
Measurements.—Holotype, length 6.3, midwidth 8.4, hinge width 8.2, thick- 
ness 2.8. 
Types——Holotype: 110916a; figured paratype: 110916b; unfigured para- 
types: II0QI2, 110916c. 
Horizon and locality—Arline formation in Tennessee: In glade 4 mile south- 
east of Friendsville, Concord (T.V.A. 138-SW) Quadrangle. 
Discussion—lIsophragma subabbreviatum in outline and profile differs from 
other known species. It is more quadrate than any known form, and the median 
ridge is strongly and abruptly turned toward the pedicle valve. 
ISOPHRAGMA SULCATUM Cooper, new species 
Plate 170, B, figure 16 
Shell small, planoconvex in lateral and anterior profiles, wider than long with 
the hinge forming the greatest width ; cardinal extremities alate; anterior margin 
truncated ; anterolateral extremities narrowly rounded; lateral margins nearly 
straight, oblique. Surface multicostellate, costellae of unequal size, about 3 to 4 
in a millimeter at the front margin. 
Pedicle valve with low narrow median fold and slightly convex flanks ; inter- 
area long, apsacline. Brachial valve deeply sulcate at the umbo with sulcus ex- 
tending to the front margin, narrow and moderately deep throughout its length. 
Sides of shell nearly flat. Interarea procline. Brachial interior with septa nearly 
reaching the front margin. 
Measurements in mm.—Holotype, length 5.9, brachial length 4.8, width 18.7, 
hinge width 11.6, thickness 1.4. 
Type.—Holotype: 110917. 
Horizon and locality.—“Lenoir” formation (lowest beds with Christiania just 
above the Lenoir [with Maclurites] and below the Pratt Ferry formation) in 
Alabama: 0.15 mile southeast of Pratt Ferry, Blocton (15’) Quadrangle. 
Discussion.—This species differs from others herein described by its flat and 
deeply sulcate brachial valve. The single known specimen shows also the 2 
median septa as shallow, slightly divergent plates extending almost to the front 
margin. In this respect the species differs from other known forms in which the 
median septa extend scarcely beyond the middle of the valve. 
