PART I CHAZYAN AND RELATED BRACHIOPODS—COOPER 421 
Discussion.—This species is characterized by its parallel sides, the low con- 
vexity of the pedicle valve, and the fairly strong convexity of the opposite valve. 
In these respects it differs from all other described species of Multicostella. 
MULTICOSTELLA PLANOSULCATA Cooper, new species 
Plate 62, A, figures 1-5 
Shell large for the genus, wider than long, subrectangular in outline; hinge 
slightly wider than the midwidth; cardinal extremities slightly acute; sides 
straight, slightly oblique; front margin broadly rounded; anterior commissure 
gently and broadly uniplicate; surface multicostellate, costellae approximately 
equal in width to the striae ; 8 costellae in 5 mm. at the front margin. 
Pedicle valve unevenly convex in lateral profile, the posterior half moderately 
convex, the anterior half flattened. Anterior profile broadly and gently convex. 
Umbo somewhat swollen, not narrowly plicated as in many species of the genus ; 
fold indistinct on the posterior half, obsolete in the anterior half; front flattened 
and depressed ; flanks flattened to gently swollen; posterolateral region depressed 
and deflected toward the brachial valve; posterolateral slopes long and gentle. 
Interarea short, curved, apsacline. Interior with short and narrow muscle field. 
Brachial valve gently but somewhat unevenly convex, the greatest convexity 
at about the middle, the anterior somewhat flattened; anterior profile broadly 
and gently convex ; umbo sulcate, sulcus short and narrow, disappearing at about 
the middie; median region gently swollen; flanks level with median region and 
sloping gently to the lateral and posterolateral areas. 
Measurements in mm.—Holotype, length 22.6, brachial length 22.4, width 
27.6, hinge width 28.0, thickness 8.6. 
Types.—Holotype: 110423a; figured paratype: 110412a; unfigured paratypes: 
I10412b,c, 104423b-1. 
Horizon and locality —Arline formation in Tennessee: 1 mile east-northeast of 
Friendsville, Louisville (T.V.A. 138-SE) Quadrangle; north side of wagon road 
in glade, + mile southeast of Friendsville, Concord (T.V.A. 138-SW) Quad- 
rangle. 
Discussion.—This species is suggestive of M. plicata but differs in not having 
the strong median fold of the pedicle valve, the deep sulcus of the brachial valve. 
It is reminiscent of M. quadrata in size but is more strongly costellate, differently 
shaped, and somewhat less convex. Furthermore, the muscle field of M. quadrata 
is proportionately larger than that of the Friendsville species. 
MULTICOSTELLA PLATYS (Billings) 
Plate 68, A, figures 1-4 
Orthis platys Brtt1ncs, Canadian Nat. Geol., vol. 4, p. 438, fig. 15, 1859; Geol. Canada, p. 129, 
fig. 54, 1863.—HALL and CLarKE, Pal. New York, vol. 8, pt. 1, p. 218, 1892.—LEsLEy, 
Geol. Surv. Pennsylvania, Rep. P. 4, p. 530, 1889.—WINCHELL and ScHUCHERT, Geol. 
Minnesota, vol. 3, p. 424, 1893. 
Dinorthis platys (Billings) ScHucHERrT, U.S. Geol. Surv. Bull. 87, p. 216, 1897. 
