PART I CHAZYAN AND RELATED BRACHIOPODS—-COOPER 311 
biconvex, the pedicle valve with the greater convexity. Surface marked by 
strong angular costae numbering about 16 to 18. 
Pedicle valve gently convex in lateral profile and with the maximum convexity 
in the midregion ; anterior profile broadly convex but with the median region 
narrowly rounded and slightly elevated ; umbo narrowly swollen, the swelling con- 
tinued to the front margin as a poorly defined fold; lateral slopes flat and gently 
sloping ; interarea short, curved, gently apsacline ; beak incurved. 
Brachial valve very gently convex with maximum convexity at the middle; 
anterior profile broadly convex but with a narrow, V-shaped depression in the 
middle; sulcus originating at the umbo widening anteriorly to the margin where 
it occupies slightly more than one-third the shell width; flanks bounding sulcus 
gently swollen; posterolateral extremities slightly depressed. 
Measurements in mm.— 
Brachial Hinge 
Length length Width width Thickness 
PIGUICY PE ces, outclsihs a seh 8.0 7.5 9.6 9.4 4.0 
Paratype (109802a)..... 6.5 6.0 7.9 8.1 3.5 
Types.—Holotype: 116877; paratype 1o9802a. 
Horizon and locality—Shippensburg formation (Pinesburg member-just 
above Erhinosphaerites zone) in Maryland: On U. S. Highway 40 at Wilson, 
west side Conococheague Creek, 7 miles west of Hagerstown, Williamsport (15’) 
Quadrangle. 
Discussion.—This species is similar to O. blountensis in shape but differs in 
having a less convex brachial valve, more elongate interarea, less convex pedicle 
umbo, and less swollen pedicle valve. 
ORTHAMBONITES ROTUNDIFORMIS Cooper, new species 
Plate 34, F, figures 27-39 
Small to medium size for the genus, thick-shelled; length and width nearly 
equal or with the length exceeding the width. Lateral margins straight to slightly 
curved ; anterior margin narrowly rounded. Hinge equal to or slightly less than 
the greatest shell width. Cardinal extremities slightly obtuse or forming a right 
angle. Surface marked by 18 to 24 low, subangular costae separated by spaces 
wider than the costae. 
Pedicle valve with the maximum convexity in the posterior half when viewed 
from the side but in anterior profile strongly convex with slightly flattened flanks. 
Posterior margin forming an angle of 120°. Beak strongly incurved ; umbo and 
median part swollen and convex with flattened but steep slopes to the lateral 
margins. Anterior slope less steep than the lateral ones. Interarea long, gently 
apsacline. Teeth large with deep fossettes; dental plates stout, short; umbonal 
cavities filled by callus. Muscle area confined to the delthyrial cavity. 
Brachial valve strongly convex in lateral profile with the maximum curvature in 
the posterior half and with the front half flattened. Anterior profile strongly con- 
vex but with the crest of the curve slightly flattened. Sulcus poorly defined, origi- 
nating at the umbo. Flanks swollen and with steep lateral slopes. Posterolateral 
