PART I CHAZYAN AND RELATED BRACHIOPODS—COOPER 433 
CHAULISTOMELLA BREVIS (Willard) 
Plate 65, B, figures 7-12; plate 66, B,C, figures 10-27 
Plaesiomys brevis WiLiarp, Bull. Mus. Comp. Zool., Harvard Coll., vol. 68, No. 6, p. 268, 
pl. 1, figs. 12, 14, 1928. 
P. platys Witiarp (not Billings), idem, p. 266, pl. 1, figs. 10, 11, 1928. 
Shell subrectangular in outline, wider than long, hinge nearly a right angle 
or extended slightly to have a width greater than that at the middle; cardinal 
extremities often forming small ears; sides nearly straight or gently rounded ; 
anterior margin broadly rounded ; anterior commissure rectimarginate to slightly 
and broadly uniplicate. Surface multicostellate, costellae not strongly differenti- 
ated, 10 to 14 in the space of 5 mm. at the front margin. Concentric fila strongly 
developed, about 4 or 5 to the millimeter at the middle. 
Pedicle valve unevenly convex in lateral profile, the posterior half very gently 
convex but the anterior half gently concave. Umbonal region gently swollen, 
somewhat narrowly swollen medianly with the swelling extended anteriorly as 
a more or less well developed narrow fold. Anteromedian region flattened or 
gently concave; lateral extremities deflected moderately in the direction of the 
brachial valve. Interarea moderately long, strongly apsacline. Interior with 
small, moderately indented muscle field; pallial trunks not strongly developed. 
Brachial valve moderately convex in lateral profile with the greatest con- 
vexity in the posterior two-thirds; anterior third somewhat flattened. Anterior 
profile broadly and moderately convex; median region swollen; umbo gently 
sulcate, sulcus disappearing before the middle; anterior and lateral slopes mod- 
erately steep, fairly short. Interarea gently apsacline. Interior with short, thick 
median ridge, thick notothyrial platform, and stout cardinal process. 
Measurements in mm.— 
Brachial Hinge 
Length length Width width Thickness 
Hypotype (116995a)......... 25.1 26.1 31.2 30.5 8.2 
= (i16904e)'..0.o. 20.4 21.0 25.2 22.8 6.8 
¥ CTIOAG2D Pett. SAR 17.4 18.0 21.0 20.7 6.0 
Types.—Holotype: M.C.Z. 8600c ; figured hypotypes: 110462b,d, 116994a-e, 
1169952. 
Figured specimen.—Plaesiomys platys Willard (not Billings): M.C.Z. 8598. 
Horizon and locality—Benbolt formation and Benholt part of Dryden forma- 
tion in Tennessee: 14 miles west of Lone Mountain; 14 miles northeast of Lone 
Mountain, Tazewell (T.V.A. 154-NE) Quadrangle; 1.1 miles northeast of Lee 
Valley, Lee Valley (T.V.A. 171-NW) Quadrangle; roadside 14 miles north- 
east of Heiskell, Powell Station (T.V.A. 137-SE) Quadrangle; Liberty Hill 
north of Lutrell, Dutch Valley (T.V.A. 154-SE) Quadrangle; hillside south of 
Dutch; 4 mile southwest of Fleanor Mill, Powell Station (T.V.A. 137-SE) 
Quadrangle. 
Benbolt formation in Viriginia: At Fugates Hill north of Mendota, Bristol 
(30) Quadrangle; south side of Mount Hagan School, southeast corner of the 
northeast subquad., Hilton (T.V.A. 197-NW) Quadrangle; 100 feet above 
