448 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 127 
anterior margin where it is occupied by about 4 costae. Flanks bounding sulcus 
gently swollen with short and gentle slopes to the margins. 
Measurements in mm.—Holotype, length 12.7, brachial length 12.3, width 15.0, 
hinge width 11.2, thickness 5.9. 
Types.—Holotype: 117019a; unfigured paratypes: 117019b-f. 
Horizon and locality.—Little Oak formation in Alabama: From a cut on U. S. 
Highway 31, 4 mile north of Pelham, Bessemer Iron District (15’) Quadrangle. 
Arline formation in Tennessee: Along wagon road in glade, + mile southeast 
of Friendsville, Concord (T.V.A. 138-SW) Quadrangle. 
Discussion.—This species is characterized by its subcircular outline, the scat- 
tered and irregular intercalation, and the elevated, narrowly rounded costellae. 
It is most suggestive of P. compacta from Tennessee but differs in having a much 
more compressed profile, more slender costae, and less convex valves. A few 
specimens from Friendsville, Tenn., agree with the Alabama species in the slender 
character of the costae. 
PLECTORTHIS COMPACTA Cooper, new species 
Plate 84, B, figures 6-17 
Shell of moderate size for the genus, width about one-quarter greater than 
the length. Hinge narrower than the greatest shell width and about equal to 
the length. Greatest width at about the middle. Cardinal extremities obtuse ; 
lateral margins narrowly rounded; anterior margin gently rounded to nearly 
straight. Surface costate, with 35 to 38 costae. The costae on the flanks turn 
posterolaterally at about two-thirds their length from the beak. In the region 
of the cardinal extremities a few costae are intercalated, and occasional costae 
are intercalated on the body of the shell. 
Pedicle valve strongly and evenly convex in lateral profile; broadly and gently 
convex in anterior profile. The greatest convexity at the middle in both profiles. 
Beak short and stout, incurved ; umbo swollen; umbonal slopes moderately steep ; 
gently concave. Flanks and middle part gently swollen; fold defined only as a 
gentle median swelling, usually inconspicuous. Interarea short, moderately 
apsacline ; slightly curved. Muscle field heart shaped, dental plates well developed. 
Brachial valve slightly less convex and deep than the pedicle valve; umbo 
moderately swollen. Sulcus originating on the umbo and extending to the front 
margin or scarcely defined. Sulcus, when present, shallow, widening anteriorly 
and occupied by 3 to 6 costae. Flanks moderately swollen with moderately steep 
and short slopes to the lateral margins and cardinal extremities. Interarea short 
and curved, generally apsacline. Notothyrial cavity deep, cardinal process high, 
slender, median ridge reaching the middle ; brachiophores short, curved ; brachio- 
phore supports well developed. 
Measurements in mm.— 
Brachial Hinge 
Length length Width width Thickness 
PTGIOEVDE sass bares Ee Pie ta es 12.3 Diy 15.3 11.9 7.4 
Paratype '(11082Is) 007... 15.3 14.5 18.5 14.0 9.1 
re GrtoSert) att, 14.0 13.7 17.6 13.4 9.0 
3 CTIOSZTIE) o.. oe 5 cisin ete 13.4 12.9 16.2 0.7 78 
