784 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 127 
Lebanon limestone of the central basin of Tennessee. It is characterized by its 
transversely subrectangular outline, the well-differentiated costellae, and the mod- 
erately thickened interiors. 
This species name first appeared in Dr. Bassler’s description of the Stra- 
tigraphy of the Central Basin of Tennessee. In that publication the name proved 
to be a nomen nudum because the illustrations were unaccompanied by a de- 
scription. Three years later a short characterization was published, but this 
failed to indicate a type specimen. In the original publication two specimens were 
figured, and these were again referred to in the later characterization. These 
two specimens are thus the cotypes. It seems best here to select the pedicle 
valve (fig. 8) U.S.N.M. 80487a. This is a most unfortunate specimen to repre- 
sent the species because it is crushed and has an unusual development of wrinkles 
on the posterior margin. It is better, however, than the other alternative which 
is a brachial interior. 
Bassler’s characterization stressed the ornamentation and the fact that the 
septa of the brachial valve almost meet the anterior margin. This seems to be 
true in a few specimens, but generally the brachial plates terminate in a crest 
near the valve middle as they do in all other species. 
Sowerbyella lebanonensis, because of its considerable width, should not easily 
be confused with other Sowerbyellas described in this report. It is in that re- 
spect unlike S. compacta and S. socialis, which might be confused with it. The 
latter, moreover, is much more coarsely ornamented, and in the former the cos- 
tellae are not arranged in sectors of fine and large ones as in S. lebanonensis. 
SOWERBYELLA MEDIOPLICATA Cooper, new species 
Plate 204, A, figures 1-5 
Small, semielliptical in outline ; hinge forming the widest part, auriculate ; sides 
oblique; anterior margin broadly rounded; costellae subequal, about 4 to the 
millimeter at the front margin. 
Pedicle valve moderately convex in lateral profile with the maximum con- 
vexity at about the middle; anterior profile broadly convex with the median 
region narrowly humped and the flanks depressed below it; lateral slopes mod- 
erately steep. Umbonal and median regions narrowly swollen to form a marked 
median fold which forms a wave in the commissure; flanks moderately swollen. 
Brachial valve with a prominent median sulcus extending from umbo to an- 
terior margin; flanks fairly deeply concave but posterolateral extremities some- 
what flattened. Interior with slender median plates; anteromedian region mod- 
erately thickened. 
Measurements in mm.—Holotype, length 4.8, brachial length 4.3, midwidth 
7.4, hinge width 8.7, thickness 1.3, height 1.8. 
Types.—Holotype: 117498. 
Horizon and locality—Upper Lincolnshire formation in Virginia: Along 
Tumbling Run, 14 miles southwest of Strasburg, Strasburg (15’) Quadrangle. 
Discussion.—This species is characterized by small size, fairly strong but 
