496 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 127 
Types.—Figured hypotype: 117074; measured hypotypes: 117074a,c. 
Horizon and locality.—Lebanon limestone in western Tennessee: At Colum- 
bia, Maury County ; 3 miles east of Murfreesboro, Rutherford County ; Lavergne, 
Rutherford County ; cut on U. S. Highway 241, 2.1 miles north of the Bedford- 
Rutherford County line in Rutherford County; behind Roy Holt’s gasoline sta- 
tion on Tennessee Highway 10, 1.2 miles south of Lebanon town limits, Wilson 
County. 
Discussion.—This species is abundant in the Lebanon limestone of the Central 
Basin of Tennessee but is rare elsewhere. It can be recognized by the moderate 
convexity of the brachial valve, the strong median sulcus, but more particularly 
by the fasciculate arrangement of the costae on the fold. This type of fascicula- 
tion is common in many of the species of this genus, and it is to those species that 
S. halli must be compared. 
Skenidioides halli is about the same size as S. platys, but the Lebanon lime- 
stone species possesses a more convex profile in both valves. Skenidioides okla- 
homensis is suggestive of the West Tennessee species but is somewhat bigger, 
more angular, with a stronger fold and sulcus and stronger costae. 
SKENIDIOIDES MEDIOCOSTATUS Cooper, new species 
Plate 97, C, figures 19-37 
Shell fairly large for the genus, with the hinge forming the widest part. Lateral 
margins sloping toward the middle; anterolateral extremities broadly rounded 
and the anterior margin truncate. Anterior commissure strongly sulcate. Pro- 
file planoconvex. Ornamentation variable, a broad central costa flanked by 9 
to 12 costae. Most costae primary ; intercalated costae few and occurring chiefly 
on each side of the median costa. 
Pedicle valve slightly convex in lateral profile; more strongly convex in an- 
terior profile. Umbo slightly convex; fold increasing in prominence gradually 
to the anterior margin. Flanks flat in profile, their slope moderate. Interarea 
long, nearly flat and nearly procline. Spondylium broad and shallow. 
Brachial valve nearly flat in all profiles; sulcus originating at the umbo and 
extending to the front margin where it occupies somewhat less than half the 
width ; 5 to 6 costae occupy the sulcus. Flanks bounding sulcus slightly convex. 
Region between flanks and cardinal extremities very slightly concave. 
Measurements in mm.— 
Brachial Hinge 
Length length Width width Thickness 
TIGIGE DE, seisink wlalaiaiaus’ ce 3.0 2.5 4.0 4.5 2.0 
Paratype (117075b)..... 2.6 2.5 4.0 4.7 1.5 
‘s (TI7O75a) LU 2.5 2.6 3.9 4.4 na 
Types.—Holotype: 117076a; figured paratypes: 117075a-c, 117077¢,d,j; un- 
figured paratypes: 117076b, 117077a,b,e-i,k,l. 
Horizon and locality.—Benbolt formation in Virginia: In the vicinity of Rye 
Cove, Clinchport (T.V.A. 188-NW) Quadrangle; 0.2 mile south of the middle 
fork of Moccasin Creek, 14 miles due south of Hansonville, Hansonville (T.V.A. 
