402 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 127 
tus is stronger in both lateral and anterior directions. The sulcus of the brachial 
valve of D. atavoides is stronger than that of D. venusta. 
DINORTHIS VIRGINIENSIS Cooper, new species 
Plate 59, B, figures 17-21 
Shell unusually large for the genus, subrectangular in outline with the length 
less than the width; hinge narrow; cardinal extremities obtusely rounded; sides 
broadly rounded; front margin broadly truncated; surface multicostate, costae 
numbering about 4 in 5 mm. at the front margin. 
Pedicle valve nearly flat to gently concave with the most concave part in the 
median region ; umbonal region gently convex; fold originating at the umbo, low 
and inconspicuous throughout its length to the front margin; fold a narrow 
median fascicle of 3 primary costae expanding and increasing anteriorly by 
bifurcation and implantation. Flanks bounding fold gently concave; lateral and 
posterolateral regions flat. Interarea short, apsacline. Interior with long, sharply 
defined muscle field longitudinally rectangular in outline but with the anterior 
margin deeply indented ; adjustor scars prominent ; diductor scars long, oblique, 
sharply pointed at the anterior ends; adductor ridge elevated ; pallial marks not 
strongly impressed. 
Brachial valve fairly strongly convex in lateral profile with the maximum con- 
vexity at about the middle ; anterior profile strongly and broadly convex, with the 
top somewhat flattened but with short, steep slopes to the sides; broadly sulcate ; 
flanks bounding sulcus swollen; posterolateral slopes short and steep. 
Measurements in mm.— 
Brachial Hinge 
Length length Width width Thickness 
Holott be: is cammcs’ oy) Jeatavanl up ot. « 29.9 30.4 38.7 29.0 115+ 
Paratype (pedicle valve 116980b).... 26.0 ? 34.0 29.0 ? iy 
Types.—Holotype: 116979; figured paratypes: 116980a,b; unfigured para- 
types: 116980c,d. 
Horizon and locality—Edinburg formation (50 to 75 feet above cherty bed 
of Lincolnshire) in Virginia: From a quarry on the north side of U. S. Highway 
33, I mile west of the junction with Virginia Highway 613, 4 miles west of Dale 
Enterprise, Harrisonburg (15’) Quadrangle ; 500 to 600 feet above the “Lenoir” 
on Colliers Creek on Virginia Highway 251, $ mile north of the junction with 
Virginia Highway 612, 6 miles west-southwest of Lexington, Rockbridge County. 
Discussion.—This is the largest species of Dinorthis at present known. The 
ornamentation of the pedicle valve indicates clearly that it is related to D. trans- 
versa. It differs in its great size and much less transverse outline. 
DINORTHIS WILLARDI Cooper, new species 
Plate 59, A, figures 1-16 
Shell large for the genus, wider than long with obtusely rounded cardinal ex- 
tremities and broadly rounded lateral and anterior margins. Anterior commis- 
