PART I CHAZYAN AND RELATED BRACHIOPODS—COOPER 259 
Posterior margin straight and characterized by a pseudointerarea of considera- 
ble length in old shells; pseudointerarea often growing as a shelf over posterior 
end of median septum. Two large muscle scars occupy the posterolateral areas 
on each side of the median septum. 
Genotype.—Rhysotreta corrugata Cooper, new species. 
Discussion.—This pretty little genus is characterized by its concave brachial 
valve, the strongly corrugated exterior, and the fairly large tear-shaped pedicle 
opening. Unfortunately, none of the numerous specimens obtained by solution 
of limestone by acetic acid shows any details of the muscle system of the pedicle 
valve. 
The inside of the brachial valve is characterized by a strong median septum 
that rises to its highest point at the anterior end of the valve. This septum origi- 
nates near the middle of the valve in very young specimens. The small brachial 
valve is also deeply concave, but as it grows, the anterior thickens and levels off 
to form a flattened anterior rim. This rim in old shells may grow toward the 
pedicle valve and produce a strongly convex rim. 
RHYSOTRETA CORRUGATA Cooper, new species 
Plate 18, F, figures 37-50 
Shell small, thick, forming a cone with flattened posterior ; surface marked by 
strong, rounded concentric rings. 
Pedicle valve with anterior margin gently convex; posterior margin flat to 
gently convex; lateral slopes strongly rounded ; anterior profile with very gently 
convex sides; apical angle about 30°, pseudointerarea broad, flat but marked 
by concentric corrugations, intertrough variable, broad to narrow but always 
shallow ; beak blunt ; foramen large, located on the posterior side of the beak. 
Brachial valve with nearly straight hinge line; beak small, protruding beyond 
the posterior margin ; lateral profile flat to slightly concave ; anterior profile gently 
concave ; median region deeply concave. 
Measurements in mm.— 
Length Width Thickness 
Holotype (pedicle valve), °... .{s:s:1* 250-4 -ulaoet = ge) 1.6 1.0 
Paratype (brachial valve 116817d) ........... 1.4 1.9 0.3 
Types.—Holotype: 116817a; figured paratypes: 116817b-g. 
Horizon and locality—Pratt Ferry formation (lower 3 feet) in Alabama: 0.2 
mile southeast of Pratt Ferry, Blocton (15’) Quadrangle. 
Discussion——No other species is now known with which this one can be 
compared. 
SCAPHELASMA Cooper, new genus 
(Greek scaphos, hollow; elasma, plate) 
Shell small, elliptical in outline, conical in profile; surface marked by fine con- 
centric lines and in the anterior half by imbricating lamellae. 
Pedicle valve symmetrically conical in anterior profile but forming a cone 
