868 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 127 
is at the middle; sides gently rounded; anterior margin strongly rounded; cos- 
tellae fine and delicate, 4 to a millimeter at the front margin. Costellae on pos- 
terior half alternating strong and fine, as many as 7 fine costellae separating the 
strong ones; anterior with costellae not distinctly differentiated. 
Pedicle valve gently concave as usual in the genus; muscle region moderately 
large; ridges extended anterior to dental ridges unusually short. 
Brachial valve unevenly convex in lateral profile and with the posterior quar- 
ter gently concave and the anterior three-quarters moderately convex ; maximum 
convexity slightly anterior to the middle; anterior profile moderately and broadly 
convex. Umbonal region gently concave; anterior three-quarters swollen medi- 
ally; lateral slopes short and steep; anterior slope moderately long and steep; 
cardinal extremities rounded and deflected in the direction of the brachial valve. 
Measurements in mm.—Holotype, length 32.8, brachial length 31.0, midwidth 
34.1, hinge width 27.8?, thickness ?. 
Type.—Holotype: 117671a; figured paratype: 117671b. 
Horizon and locality—Sevier formation in Tennessee: 1 mile by road north- 
west of Big Springs, Binfield (T.V.A. 139-NE) Quadrangle. 
Discussion.—This species is known from two specimens which reveal the 
pedicle interior, the brachial exterior, and the interarea of the pedicle valve. The 
species is most like R. tennesseensis but differs notably in its profiles. The lateral 
profile of R. tennesseensis is much more convex in the region of geniculation 
than the Sevier species. Furthermore, the concave or depressed region from the 
brachial beak to the place of geniculation is deeper and longer in FR. tennesseensts. 
This area in the Sevier species is only depressed for a short distance in the 
vicinity of the umbo. The median region is marked by a broad swelling rather 
than a marked geniculation. In anterior profile R. filicostellata is gently and 
broadly convex, but R. tennesseensis is strongly domed. 
RHIPIDOMENA MESLERI Cooper, new species 
Plate 218, C, figures 8-10; plate 240, C, figures 13-17; plate 250, A, figures I-14; 
plate 254, A, figures 1-3 
Shell of moderate size for the genus, wider than long with the width about 
14 times the length; semielliptical in outline; cardinal extremities varying from 
acutely angular to obtuse depending on age, the older specimens having right 
or obtuse angles. Lateral margins variable but usually straight or nearly so in 
the adults. Anterior and anterolateral margins broadly rounded. Surface cov- 
ered by costellae of unequal size; the larger or primary costellae are separated 
by 1 to 3 finer ones. All costellae widen slightly anteriorly but in some speci- 
mens tend toward uniformity in size. In 5 mm. at the front of 2 specimens, 21 
and 23 costellae may be counted. Costellae are crossed by elevated fila about 11 
to a millimeter at about the middle of a valve 20 mm. long. 
Pedicle valve very gently to moderately concave at about the middle ; umbonal 
region very gently swollen; anterior half very gently concave to nearly flat. 
Cardinal extremities only slightly deflected toward the brachial valve. Lateral 
