PART I CHAZYAN AND RELATED BRACHIOPODS—COOPER 451 
PLECTORTHIS PENNSYLVANICA Cooper, new species 
Plate 92, G, figures 30-44 
Shell of about medium size for the genus, wider than long; hinge narrower 
than the greatest width which is at the middle; sides narrowly rounded; anterior 
margin broadly rounded; anterior commissure rectimarginate. Surface marked 
by 29 narrowly rounded costae equal in width to the width of the spaces between 
them. Interspaces marked by strong, distant fila. 
Pedicle valve with gentle lateral profile having the maximum convexity at 
about the middle ; anterior profile broadly convex and moderately humped in the 
middle ; umbo somewhat narrowly swollen, the swelling extending to the middle 
of the valve but lessening anteriorly ; flanks and lateral areas descending gently 
to the margins. Interarea, short, curved, strongly apsacline. 
Brachial valve very gently convex in lateral and anterior profiles; umbo sul- 
cate, sulcus shallow from umbo to margin but widening broadly; sulcus occu- 
pied by 4 costae. Flanks bounding sulcus gently swollen; lateral slopes long 
and gentle. 
Measurements in mm.—Holotype, length 10.6, brachial length 9.7, width 13.7, 
hinge width 10.9, thickness 4.9. 
Types.—Holotype: 11702I1a; unfigured paratype: 117021b. 
Horizon and locality—Rodman member of Nealmont formation in Pennsyl- 
vania: On the east side south quarry at Sparr, Huntingdon (15’) Quadrangle. 
Discussion.—This species suggests the P. plicatella group of the higher parts 
of the Trenton limestone. It differs from that species, however, in its proportion- 
ately wider hinge, less convex valves, and more closely spaced costae. It is a 
larger species than P. symmetrica with wider hinge, somewhat less numerous 
costae, and proportionately less convex valves. 
PLECTORTHIS PONDEROSA Cooper, new species 
Plate 83, C, figures 22-32; plate 84, E, figures 24-32 
Shell large for the genus, subquadrate in outline; hinge narrow; greatest shell 
width at the middle; sides broadly rounded; anterolateral extremities narrowly 
rounded; anterior margin gently curved; anterior commissure rectimarginate ; 
surface marked by about 30 strong, direct costae. 
Pedicle valve fairly strongly convex in lateral profile with the greatest con- 
vexity just anterior to the umbo; anterior half somewhat flattened ; anterior pro- 
file a broad arch; umbo and median region strongly swollen; lateral area swol- 
len and with steep slopes to the lateral margin; interarea strongly curved, gently 
apsacline. Muscle area small; diductor scars long and narrow; adductor track 
moderately wide. 
Brachial valve with gentle convexity in lateral profile, its depth considerably 
less than that of the pedicle valve; median region swollen; sulcus shallow and 
narrow, originating at the umbo and widening to the front margin where it is 
inconspicuous. Flanks bounding sulcus broadly swollen; lateral slopes moder- 
