PART I CHAZYAN AND RELATED BRACHIOPODS—COOPER 775 
Horizon and locality—Edinburg formation (Cyrtonotella bed) in Strasburg 
(15’) Quadrangle, Virginia: In the ravine at the switch } to + mile east of Stras- 
burg Junction; Tumbling Run, 14 miles southwest of Strasburg. 
Edinburg formation (Nidulites zone) in Strasburg (15’) Quadrangle, Vir- 
ginia: On U. S. Highway 11 about 1 mile north of Strasburg ; along Tumbling 
Run, 14 miles southwest of Strasburg. 
Shippensburg formation (50 feet above base of Pinesburg member) in Mary- 
land: On U. S. Highway 40, on the west bank of Conococheague Creek, 7 miles 
west of Hagerstown, Williamsport (15’) Quadrangle. 
Same formation in Pennsylvania: At the east end of the railroad cut and 
adjacent field to the north, 24 miles southwest of Marion, Chambersburg (15’) 
Quadrangle. 
Discussion——Moderately strong and subequal costellae characterize this 
species. The shell is generally of moderate size and fairly low convexity. It is 
most like S. cava from the Oranda and Salona formations but differs in attaining 
a somewhat greater size, interareas less bent anteriorly, and considerably less 
convexity. This is not a common species in the lower Edinburg formation. 
SOWERBYELLA AEQUISTRIATA (Willard) 
Plate 197, C, figures 25-34 
Plectambonites aequistriatus WiLLARD, Bull. Mus. Comp. Zool., Harvard Coll., vol. 68, No. 6, 
p. 276, pl. 3, figs. 8, 9, 1928. 
About medium size for the genus, wider than long, semielliptical in outline; 
cardinal extremities nearly a right angle and with the extremities drawn into 
tiny ears. Sides nearly straight to gently rounded; anterior margin broadly but 
strongly rounded. Costellate, costellae at front of pedicle valve generally alter- 
nating large and fine but on brachial valve usually more equal in size. About 
5 costellae to the millimeter on the pedicle valve and 7 to the millimeter at the 
front margin of the brachial valve. 
Pedicle valve evenly and gently convex in lateral profile, with the maximum 
convexity at about the middle; anterior profile with the median region some- 
what narrowly convex but with the sides descending in long, only moderately 
steep slopes to the margins. Beak small, umbo narrowly swollen and with the 
swelling continued anteriorly as a poorly defined fold. Flanks flat to gently con- 
cave. Interior with the diductor scars diverging at about 80° and separated by 
a short median ridge. 
Brachial valve gently concave; umbo concave, the concavity extended an- 
teriorly as a shallow and narrow sulcus; flanks gently concave. Interior with 
septa slightly divergent and forming a crest at about two-thirds the length from 
the beak. Adductor field thickened at the front. 
Measurements in mm.— Brachial Hinge 
Length length Midwidth width Thickness Height 
Hypotype (117478f).. 9.3 8.6 13.0 13.2 1.7 2.5 
‘ (117478g).. 9.9 9.1 14.8 1731 2.0 3.8 
ee (117478a).. 10.4 9.7 13.2 14.1 1.8 2.5 
