870 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 127 
pedicle valve, shorter and less concave umbonal region on the brachial valve, and 
a less sharply geniculated brachial valve. 
Rhipidomena mesleri differs strongly from R. tenuitesta in having more pro- 
nounced and more strongly convex brachial valve, shorter umbonal swelling in 
the pedicle valve, and more rectangular outline. 
RHIPIDOMENA SUBPARALLELA Cooper, new species 
Plate 251, A, figures 1-12 
Shell attaining a length of slightly over an inch; wider than long and with the 
hinge forming the widest part; sides subparallel, slightly excavated just anterior 
to the cardinal extremities. Anterior margin narrowly rounded; profile mod- 
erately convexi-concave. Finely costellate ; 3 costellae to a millimeter at the front 
margin. 
Pedicle valve unevenly concave in lateral profile, the posterior half gently con- 
vex but the anterior half gently concave; umbonal and posterior half of the 
median region gently swollen; lateral and anterior regions gently concave to 
nearly flat; interarea moderately long, pseudodeltidium short. 
Brachial valve nearly flat in the posterior half of the lateral profile but with 
anterior half bent abruptly toward the pedicle valve and gently convex. Median 
region concave ; lateral regions and anterior half gently convex and sloping mod- 
erately. Interarea short. 
Measurements in mm.— 
Brachial Hinge 
Length length Midwidth width Thickness 
FIOloty per ees whe ata dabe 28 26.8 25.8 29.3 32.1 4.6 
Paratype 1 (117681a))2.). »2 desu 24.8 24.2 25.6 26.8 4.0 
Types.—Holotype: 117679; figured paratypes: 117680a, 117681a,b; unfigured 
paratypes: 117680b, 117681c,d. 
Horizon and locality—Elway formation in Tennessee: Road cut and quarry 
on U. S. Highway 25W, 4.7 miles south of Clinton, Powell Station (T.V.A. 
137-SE) Quadrangle. 
Discussion—This species is characterized by its quadrate or subrectangular 
outline, which generally creates the illusion of a shell that is longer than wide. 
This species is likely to be confused only with R. tennesseensis, which it strongly 
resembles. It differs from the Benbolt species in having less anterior convexity 
and less pronounced geniculation. 
Rhipidomena subparallela is more strongly geniculated than FR. tenuitesta as 
it has a much more pronounced strophomenoid form than the Hogskin-Lincoln- 
shire species. 
RHIPIDOMENA TENNESSEENSIS (Willard) 
Plate 252, A, figures 1-5; plate 252, B, figures 6-14; plate 253, A, figures 1-14 
Strophomena tennesseensis WiLtard, Bull. Mus. Comp. Zool., Harvard Coll., vol. 68, No. 6, 
p. 285, pl. 2, figs. 17, 18, 1928. 
S. amploides Butts, Virginia Geol. Surv. Bull. 52, pl. 87, figs. 7-12, 1942. 
Shell large for the genus, varying in outline from nearly quadrate to semiellip- 
tical. Cardinal extremities deflected toward the brachial valve varying from a 
