798 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 127 
Interior of pedicle valve with low median ridge extending for about one-third 
the length of the valve, forked at the front; diductor-adjustor field wide and 
subflabellate ; inner surface coarsely papillose, thickened strongly in the antero- 
lateral and lateral regions, and less so at the front. Delthyrial callosity thick. 
Brachial valve with median septa low and moderately elevated at the front. 
Measurements in mm.— 
Brachial Hinge Thick- 
Length length Midwidth width ness Height 
Lectotype (pedicle valve).......... 8.7 ? 12.9 14.8 ? 3.2 
Paratype (pedicle valve 71525b).... 7.8 ? 11.5 10.9 ? 2.6 ? 
© Cu Ar RTE S ZOO) waging ? 10.4 TOT fa Ue 2.0 
Hypotype (brachial valve 117531e). ? 5.7 9.0 10.4 ? E 
Types.—Lectotype: 71525a; paratypes: 71525b-f; figured hypotypes: 
117543a-d, 117534a-d; measured hypotype: 11753Ie. 
Horizon and locality.—Carters formation in Birmingham (30’) Quadrangle, 
Alabama: On Cedar Mountain 2 miles southwest of Argo; quarry 0.3 mile north 
of Gate City. 
Discussion—The most prominent feature of this shell is the narrow median 
fold of the pedicle valve which gives the name to the species. No description of 
this species was offered when the name was proposed, the name resting on 7 
retouched pictures. The material on which the species is based is poorly pre- 
served, and most of the specimens of the exterior are partially exfoliated. The 
median fold in itself is not a very distinctive feature because a tendency to such 
fold seems to exist in nearly all the species. 
Sowerbyella subcarinata is a proportionately wider species than S. nasuta 
which is one of the most prominently folded species in the Appalachians. Of 
other somewhat carinate forms, this species is distinguished from S. negritus 
and S. plicatifera by their different ornamentation. The species is more convex 
than S. lebanonensis. Sowerbyella compacta is very close to S. subcarinata, and 
a few folded specimens are almost identical in profile and proportions to the 
Alabama shell. Folding in S. compacta is not the rule, and the specimens that 
so strongly resemble S. subcarinata are unusual ones. 
SOWERBYELLA VARIABILIS Cooper, new species 
Plate 197, D, figures 35-43; plate 202, EF, figures 27-40 
Shell of about medium size for the genus, wider than long, outline semiellip- 
tical to subrectangular ; hinge forming the widest part, usually auriculate in the 
young and young adult. Sides varying from slightly to moderately oblique; an- 
terior margin broadly rounded. Surface marked by costellae of unequal size, 
the 2 sizes alternating frequently ; 3 or 4 of the strong costellae occupy the space 
of I mm. at the front margin; I to 4 of the finer costellae occupying the spaces 
between the stronger ones. Alternation of costellae less marked on the pedicle 
valve than on the brachial valve and more pronounced on the front of the pedicle 
valve than other parts of that valve. 
Pedicle valve fairly strongly and evenly convex and with the maximum con- 
