PART I CHAZYAN AND RELATED BRACHIOPODS—COOPER 495 
Measurements in mm.— 
Brachial Hinge ‘ 
Length length Width width Thickness 
Holotype ie... dsnoteacdt. 7.9 6.7 7.3 75 4.3 
Paratype (117073b)..... 7.0 6.0 6.4 6.8 4.0 
= (LI7073C) «0.0: 5.5 4.3 5.8 6.0 3.2 
_ CELZO/30) 0am > 6.2 4.9 7.0 2.9 3.5 
Types.—Holotype: 117073a; figured paratypes: 117073b,c,e; unfigured para- 
type: 117073d. 
Horizon and locality—Martinsburg formation (lower part with Brongniar- 
tella= Salona), in Virginia: On Virginia County Road 617=910, 0.15 mile north 
of Green Mount Church, Broadway (15’) Quadrangle. 
Discussion.—This is the largest species yet found in the middle Ordovician 
rocks of the United States. It is not therefore directly comparable to any of the 
other species herein described. Young stages suggest S. rectangulatus but usu- 
ally are somewhat wider than that species and with a shallower sulcus. 
SKENIDIOIDES HALLI (Hall and Clarke) 
Plate 08, G, figure 30 
Skenidium halli Sarrorp, Geol. Tennessee, p. 287 (not defined), 1860. 
Scenidium halli Safford (in part) Hatt and CLARKE, Pal. New York, vol. 8, pt. 1, p. 242, 
pl. 7A, figs. 33-36, 1802. 
Fairly large for the genus, wider than long and with the hinge usually form- 
ing the greatest width. Valves unequally convex and of unequal depth, the 
pedicle valve having the greater depth. Anterior commissure strongly sulcate. 
Surface costate ; costae varying in number from about 25 to 35. Increase in num- 
ber of costae taking place by intercalation, most of the costae being primary ; 
intercalation most frequent near the front of the fold. Costae crowded in the 
median region. 
Pedicle valve hemipyramidal; gently convex in lateral profile and strongly 
arched to subcarinate in anterior profile. Umbo gently convex. Fold originating 
about one-fifth the length from the beak; fold low posteriorly, elevating and 
widening anteriorly to occupy about one-third the width. Flanks immediately 
adjacent to fold flat to slightly concave; slope of flanks to margins moderately 
steep. Interarea long, strongly apsacline to nearly procline. 
The young brachial valve is nearly flat, but with growth the shell becomes 
gently to moderately convex with a tendency to a weak geniculation of the 
anterior. The sulcus originates near the beak and increases rapidly in depth and 
width to the front where a short tongue is developed. As many as 9 costae may 
occupy the sulcus at the front. Flanks bounding sulcus gently swollen; areas 
between flanks and cardinal extremities gently concave. 
Measurements in mm.— 
Brachial Hinge 
Length length Width width Thickness 
Hypotype (pedicle valve 117074a)..... 3:3 ? 4.5 5.0 ? 
‘7 (brachial valve 117074c).... ? 2.6 4.4 4.2 ? 
