454 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 127 
PLECTORTHIS TENUIS Cooper, new species 
Plate 81, E, figures 18-29 
Shell large for the genus, slender in lateral profile; valves subequally biconvex 
but the pedicle valve with the greater depth; hinge narrow; greatest width at 
about the middle; sides somewhat narrowly rounded; anterior margin broadly 
rounded; anterior commissure rectimarginate. Surface marked by about 42 
narrowly rounded costae. 
Pedicle valve with moderately convex lateral profile; anterior profile broadly 
convex and not conspicuously swollen in the middle; umbonal region somewhat 
swollen; median region swollen and with long gentle slopes to the lateral mar- 
gins. Interarea short, apsacline, slightly curved; muscle field small, narrow; 
diductor impressions deeply impressed ; adductor track slightly elevated. 
Brachial valve with moderately convex lateral profile ; anterior profile broadly 
convex and with a slight median depression. Sulcus originating on the umbo, 
shallow and widening anteriorly to the front margin; occupied by 4 costae at 
the front. Flanks bounding sulcus gently swollen; posterolateral slopes short, 
moderately steep. 
Measurements in mm.— 
Brachial Hinge 
Length length Width width Thickness 
Holotype: acces eeeiees ces ok 11.8 10.4 13.7 10.4 4.5 
Paratype ¢€FE7O2Z7 Pee eee 15.8 15.6 22.3 15.0 6.8 ? 
Types—Holotype: 117028c; figured paratypes: 117027, 117028a,b,d. 
Horizon and locality—Benbolt formation (base) in Virginia: 0.2 mile south 
of the middle fork of Moccasin Creek, 14 miles due south of Hansonville, Han- 
sonville (T.V.A. 205-SW) Quadrangle; north side of U. S. Highway 19, 13 
miles southeast of Hansonville Post Office, Brumley (T.V.A. 205-SE) Quad- 
rangle; south side of Mount Hagan School, southeast corner of northeast sub- 
quad., Hilton (T.V.A. 197-NW) Quadrangle. 
Benbolt formation in Tennessee: 0.1 to 0.2 mile north of the road 1.1 miles 
northeast of Lee Valley, Lee Valley (T.V.A. 171-NW) Quadrangle. 
Discussion.—This species is characterized by its slender form and fine, numer- 
ous costae. It is a fairly wide species and is thus suggestive of P. transversa, but 
that species is more strongly costate. It is actually wider and longer than 
P. compacta but is easily differentiated by its compressed profile and slender 
costae. 
PLECTORTHIS TRANSVERSA Cooper, new species 
Plate 84, A, figures 1-5 
Shell of about usual size for the genus, subelliptical in outline, lenticular in 
profile, with narrowly rounded lateral margins and a very gently rounded an- 
terior margin. Hinge equal in width to about two-thirds the maximum width 
which is at the middle. Surface ornamented by 34 narrow, elevated, rounded 
costae which appear in 3 generations by intercalation. 
Pedicle valve in lateral profile most convex on the umbo and slightly posterior 
