PART I CHAZYAN AND RELATED BRACHIOPODS—-COOPER 959 
and anterior margin gently rounded; anterior commissure faintly sulcate. Cos- 
tellae measuring 3 to the millimeter at the anterior margin. 
Pedicle valve gently convex in lateral profile; subcarinate in anterior profile 
with flat, moderately steep sides. Umbo and median region subcarinate, the fold 
continued to the anterior margin. Flanks bounding fold flat and moderately 
steep. Beak erect, gently incurved ; interarea long and wide, apsacline. Interior 
with large teeth, stout but short dental plates; narrow and short muscle field. 
Brachial valve gently convex in lateral profile and with the anterior quarter 
bent more or less strongly toward the pedicle valve; anterior profile broadly and 
gently convex but with the median region depressed. Umbo sulcate; sulcus 
widening and deepening anteriorly and forming a broad V. Flanks bounding 
sulcus very gently convex; posterolateral areas sloping gently. Interior with 
stout and moderately long brachiophores ; brachiophore supports obscure; cardi- 
nal process variable, small; median ridge strong. Adductor pairs subequal. 
Measurements in mm.— 
Brachial Hinge 
Length length Width width Thickness 
Paratype.) (EI730GH) oa ats = 8.2 aS Q.1 6.9 4.0 
“ CEI74GSt) . oe sas 8.1 7.6 8.5 7.1 4.1 
< CTi7 ZS) AVS. 7.8 7.4 7.7 7.1 3.9 
Types.—Holotype: 117368b; figured paratypes: 117367a, 117368c-g, 
123235a-d; unfigured paratypes: 117368a,h-j. 
Horizon and localty.—Edinburg formation (low to middle Nidulites zone) 
in Strasburg (15’) Quadrangle, Virginia: On U. S. Highway 11, 1 mile north of 
Strasburg ; beside the road along Tumbling Run, 14 miles southwest of Strasburg. 
Siltstone in the Edinburg formation (Liberty Hall facies 30 feet above Echino- 
sphaerites) in Virginia: 4 mile south of Virginia Highway 274, 14 miles south- 
west of Wadesville, Winchester (15’) Quadrangle. 
Discussion.—This species is characterized by its squarish outline, fairly wide 
hinge, fine costellae, deep sulcus, and strong, subcarinate fold. Another com- 
mon feature of this shell is that it develops strong plaits of growth at the anterior 
margin. This species differs from P. matutina in having finer costellae, deeper 
sulcus and more prominent fold, and a generally larger size. 
EREMOTREMA Cooper, new genus 
(Greek eremo, solitary; trema, opening) 
Shells small, biconvex, subcircular to subquadrate, lateral margins rounded; 
anterior margin strongly rounded ; anterior commissure rectimarginate; surface 
costellate, costellae fine, rounded, increasing by implantation in 4 generations. 
Punctate. 
Pedicle valve somewhat deeper than the brachial valve, gently convex in pro- 
file; beak small, incurved ; interarea long, nearly orthocline. Interior with thick, 
large teeth; delthyrial cavity deep and narrow; muscle area rhomboidal in out- 
line ; muscle scars deeply inserted, diductors long and narrow with their anterior 
ends approximate. Adductor track long and slender, obscure; pallial impressions 
