278 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 127 
elevation and distance, occasionally fairly regular, more often slightly wavy; 
ridges occasionally attaining 4 mm. or more in height. 
Pedicle interior: Foramen entering valve near the posterior margin; slit form- 
ing a low ridge on inside anterior to foramen and extending into umbonal cavity. 
Muscles not visible in material at hand. 
Brachial interior with posterior margin thickened and incurved ; inner surface 
of posterior margin with a crescent-shaped mark convex inward. Lateral and 
anterior margins flat. Center of valve occupied by 2 large muscle scars, oval in 
outline and with their small ends directed posterolaterally. Anterior occupied 
by 2 narrowly diverging thin pallial trunks. 
Growth—yYoung specimens definitely oval in outline and marked by fine, 
somewhat distant elevated ridges. Posterior slope lengthened only slightly. 
Measurements in mm.— 
Length Width Thickness 
Paratype (pedicle valve 116808a) ........... 6.5 5.6 ? 
2 (brachial valve 117972b) .......... 10.0 9.0 0.75? 
Types—Holotype: 116808-1; figured paratypes: 116808a,c-e,g-k ; 117972a,b ; 
unfigured paratypes: 116808b,f, 117972c-e. 
Horizon and locality—Pratt Ferry formation in Alabama: 0.2 mile southeast 
of Pratt Ferry, Blocton (15’) Quadrangle. 
Discussion.—The ornamentation with its highly elevated ridges is distinctive 
in this species and unlike any other known. The development of the ornamenta- 
tion is interesting. On the anterior slope, continuous ridges alternate with short 
ridges apparently formed at the front of the valve. These short ridges die out 
on the flanks. 
SCHIZOTRETA MICROTHYRIS Cooper, new species 
Plate 20, A, figures 1-3; plate 22, D, figures 6-9; plate 22, I, figure 16; plate 27, A, figures 1-3 
Shell large, nearly circular in outline, the length slightly greater than the width. 
Pedicle valve in lateral profile forms a low cone with long concave anterior slope 
and shorter, steeper, but flat posterior slope. Apex located somewhat less than 
one-third the length from the posterior margin. Beak small and erect. Pedicle 
groove short, narrowly elliptical and measuring slightly less than 2 mm. in length. 
Lateral slopes steep, very gently concave. 
Brachial valve forming an irregular low cone with the apex located about one- 
fourth the length anterior to the posterior margin. Apex low and rounded, an- 
terior slope nearly flat or gently concave; posterior slope short, concave, and 
steep. 
Surface of both valves marked by crowded, fairly regular elevated concentric 
lines. 
Measurements in mm.—The holotype, a pedicle valve, measures 21.7 in length, 
19.5 in width; the posterior slope is 8 long but the apex is 6 in a straight line 
anterior to the posterior margin. A small brachial valve (paratype 97547a) is 
12.5 long by 11.5 wide, the apex is 3 anterior to the posterior margin in a straight 
