446 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 127 
Measurements in mm.— 
Brachial Hinge 
length Length Midwidth width Height Thickness 
Pedicle valve (117000f).... ? 11.5 15.8 ? ? ? 
Brachial valve (117009a).. 9.2 ? 13.5 10.7 ? ? B18 
Figured specimens.—117009~,b,f. 
Horizon and locality—Oranda formation in Virginia: In the railroad cut 
4 mile east of Strasburg Junction, Strasburg (15’) Quadrangle. 
Family PLECTORTHIDAE Schuchert and Cooper, 1931 
Biconvex or convexi-concave Orthacea having a variable but usually some- 
what cordate muscle area in the pedicle valve ; brachial valve having the brachio- 
phores supported by convergent plates which unite with the floor of the valve 
on each side of the cardinal process. Sockets defined by fulcral plates. Cardinal 
process simple and usually having a compressed, crenulated myophore. Del- 
thyrium and notothyrium usually unmodified. 
Subfamily PLEcTORTHINAE Schuchert, 1929 
Coarse and fine-ribbed Plectorthidae having wide hinge lines and unequal 
interareas. 
Genus DESMORTHIS Ulrich and Cooper, 1936 
Desmorthis UtricH and Cooper, Journ. Paleont., vol. 10, No. 7, p. 624, 1936; Geol. Soc. 
Amer. Special Pap. 13, p. 158, 1938. 
DESMORTHIS COSTATA Cooper, new species 
Plate 50, J, figures 42-46 
Shelli of about usual size for the genus, wider than long, with the greatest width 
at about the middle; sides gently rounded; anterior margin broadly rounded; 
anterior commissure rectimarginate. Surface costae, 36 along the margin of an 
adult shell. One generation intercalated near the middle; costae narrow, sepa- 
rated by spaces about equal in width to the width of the costae. 
Pedicle valve gently convex in lateral profile with the greatest depth located 
in the postumbonal region; anterior profile fairly strongly convex with the 
median region somewhat narrowly rounded; umbonal and median regions swol- 
len; lateral areas full and with short, steep slopes to the margins. Interarea 
long, curved, apsacline. 
Brachial valve with most convex part in the posterior region when viewed in 
lateral profile; anterior profile broadly but gently convex. Umbo swollen; sul- 
cus originating at the umbo, shallow, narrow but extending to the front margin. 
Flanks bounding sulcus gently swollen; slopes to posterolateral extremities short 
and steep. 
Measurements in mm.—Holotype, length 5.9, brachial length 5.2, width 7.3, 
hinge width 5.7, thickness, 2.5. 
Types.—Holotype: 110813a. 
