800 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 127 
generation is almost of the same size as those of the first generation. These 2 
generations of costellae separate 2 to 7 of the finer costellae; 14 to 16 costellae 
may be counted in 2 mm. at the front of a specimen of average size. 
Pedicle valve of even and moderately strong convexity ; most convex portion 
at about the middle. Anterior profile unevenly convex with the greatest con- 
vexity at the middle and with the sides flat and sloping moderately to the mar- 
gins. Beak inconspicuous, protruding very slightly beyond the posterior margin. 
Umbo narrowly swollen and passing into the swollen midregion. Slopes to the 
cardinal extremities very slight concave, gentle. Anterolateral regions some- 
what swollen, occasionally with an obscure plication defined by the strong cos- 
tellae. Interarea slightly curved, apsacline. 
Brachial valve moderately convex with the greatest convexity located at about 
the middle and decreasing anteriorly and laterally. Flanks bounding most con- 
cave portion gently convex and with the convexity decreasing most rapidly lat- 
erally. Cardinal extremities deflected slightly toward the pedicle valve. Inter- 
area short and hypercline. 
Interior: Pedicle valve with median ridge short, extending for about one- 
quarter the length; diductor scars deeply impressed, strongly thickened along 
their inner margins. Outer margins strongly thickened. Internal surface strongly 
spinose. Brachial valve with median ridge unusually strong; lateral septa low; 
none of the septa strongly elevated at the front. Median elevation of valve mod- 
erately strong, thickened at the front and with several spinous processes project- 
ing anteriorly from the front. 
Measurements in mm.— 
Brachial Hinge 
Length length Midwidth width Thickness Height 
Holotype 020). F7oeet ik. 4.6 4.0 7.0 7.5 Gy 1.9 
Paratype (117533)..... 4.9 ? Ff 8.7 ? Tigich 
. (117532e).... 4.8 4.4 7.3 8.5 1.4 1.9 
“ 1232564) iow «he 5-5 iy 8.8 11.4? ? 1.8 
- CIZI25OC) cued TE 4.9 8.7 11.8 ? ? ? 
Types.—Holotype: 117532a; figured paratypes: 117532b-d; unfigured para- 
types: 117532e, 117533, 123256a-d. 
Horizon and locality——Arline formation (middle and upper) in Tennessee: 
On both sides of the old road in the cedar glade, + mile southeast of Friendsville ; 
100 yards south of the Negro Cemetery, 4 mile northeast of Friendsville, Con- 
cord (T.V.A. 138-SW) Quadrangle; 4 mile south of McMullens, Meadow 
(T.V.A. 139-NW) Quadrangle; 3 miles southeast of Knoxville. 
Little Oak formation in Alabama: 7 to 8 miles northeast of Branchville, 
St. Clair County. 
Discussion.—This is a small species with moderately folded pedicle valve, well- 
differentiated costellae, moderately concave brachial valve, and acute cardinal 
extremities. It is distinguished from S. negritus by the lesser breadth of the 
zones of fine costellae set off by the larger radii, smaller size, and less prominent 
median folding. It is close to S. silicica but differs in having wider and more 
