426 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 127 
Brachial valve gently convex, most convex in the posterior half but flattened 
anteriorly ; anterior profile broadly but gently convex; umbo sulcate, sulcus nar- 
row and shallow to the middle, somewhat indistinctly defined in the anterior half; 
flanks moderately convex; slopes to posterolateral extremities short and fairly 
steep. 
Measurements in mm.— 
Brachial Hinge 
Length length Width width Thickness 
Flolotype oo 207. ETA 22.4 22.3 30.4 ? 8.7 
Paratype (116990) .......... 19.7 20.6 26.6 ? 25.8 ? 7.0 
i (T2RABE) ois caisyserry «(bys 11.4 wie 15.5 15.7 4.3 
Types—Holotype: 116991; figured paratypes: 116990, 123456. 
Horizon and locality.—Yellow limestone above 25-foot sandstone at base of 
Eureka group in Roberts Mountains (1°) Quadrangle, Nevada: Saddle just 
north of hill 8167, Martins Ridge, Monitor Range; 2 miles north of Martin 
Ranch. 
Discussion.—This species is characterized by its considerable width. In this 
respect it differs strongly from M. parallela which occurs in the same region. It 
is likewise wider than any of the described eastern species. 
MULTICOSTELLA ROBUSTA Cooper, new species 
Plate 63, B, figures 8-12 
Shell small for the genus, wider than long with the hinge about equal to the 
width at the middle; cardinal extremities approximately a right angle; valves 
fairly strongly and subequally deep; sides nearly straight; anterior margin 
broadly rounded; anterior commissure rectimarginate; lateral commissure 
slightly deflected toward the brachial valve in its posterior part ; multicostellate ; 
costellae closely crowded, unequal in size, about 13 in 5 mm. at the front margin. 
Pedicle valve very gently convex in lateral profile; anterior broadly convex 
with the greatest height in the middle but with long gentle lateral slopes ; umbo- 
nal and median regions moderately swollen; fold only defined at the umbo; 
lateral areas flat but gently swollen and with very gentle slopes to the margins. 
Brachial valve unevenly and fairly strongly convex in lateral profile; pos- 
terior half gently convex; anterior half somewhat abruptly flattened ; umbo swol- 
len ; umbo sulcate, sulcus scarcely defined anterior to the middle. Median region 
and flanks inflated ; slopes to posterolateral areas short and steep; posterolateral 
regions small. 
Measurements in mm.—Holotype, length 17.3, brachial length 17.2, width 20.6, 
hinge width 17.6, thickness 7.3. 
Types.—Holotype: 116992a; unfigured paratypes: 116992b,c. 
Horizon and locality.—Lower part of the Sevier formation in Tennessee: 4 
mile north of Gooseneck, Louisville (T.V.A. 138-SE) Quadrangle. 
Discussion.—This species is characterized by its fairly small size, rectangular 
form, crowded costellae, and thick cross section. It is somewhat suggestive of 
