524 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 127 
terior commissure rectimarginate ; costellae broad in early generations ; 1 in I mm. 
at the front margin; concentric fila strong. 
Pedicle valve unevenly convex in lateral profile, the posterior half gently con- 
vex, the anterior half depressed ; anterior profile broadly triangular ; median and 
lateral regions moderately swollen. Interarea long, strongly apsacline; spondyl- 
ium wide and shallow; median septum short. 
Brachial valve unevenly convex in lateral profile, the posterior half gently con- 
cave, the anterior half gently convex; anterior profile broadly and gently convex ; 
sulcus originating at the beak and extending to the anterior margin, narrow and 
moderately deep throughout its length. Umbo swollen; region anterior to umbo 
concave ; anterolateral areas bounding sulcus moderately convex ; posterolateral 
areas gently concave; notothyrial platform broad; cardinal process thin; median 
ridge fairly long. 
Measurements in mm.—Holotype, length 7.8, brachial length 7.3, width 10.6, 
hinge width 9.8+, thickness ca. 5.7. 
Types.—Holotype: 110144. 
Horizon and locality—Lincolnshire formation (Hogskin member) in Tennes- 
see: On the Maynardville Pike, 1.7 miles northeast of Hall Crossroad, Fountain 
City (T.V.A. 146-SW) Quadrangle. 
Discussion.—This species is small and has a nearly flat brachial valve, thus sug- 
gesting such species as A. ? multicostum and A. oklahomense. It differs from 
the former in having stronger and more distant costellae ; from the latter, which 
it resembles in having spiny projections on the costellae, it differs in being a much 
smaller shell and having a different outline. This species also differs from 
A. variabile in outline and ornamentation. 
ATELELASMA SULCATUM Cooper, new species 
Plate 81, B, figures 7-13 
Shell of about the usual size for the genus, wider than long; hinge slightly 
wider than the midwidth; cardinal extremities slightly auriculate; sides nearly 
straight, sloping slightly medially ; anterolateral extremities somewhat narrowly 
rounded ; anterior margin nearly straight; anterior commissure rectimarginate ; 
subequally costellate, about 10 costellae in 5 mm. at the front margin. 
Pedicle valve with uneven lateral profile, the posterior half gently convex but 
the anterior half flattened. Anterior profile broadly triangular, the sides of the 
triangle with long, moderately steep slopes ; median region slightly swollen; sides 
with long, gentle slopes; posterolateral region slightly sulcate just anterior to 
the posterior margin; interarea long, slightly procline. Spondylium narrow and 
moderately deep. 
Brachial valve moderately convex in lateral profile; anterior profile broadly 
and gently convex but median region sulcate ; median region swollen; umbo sul- 
cate; sulcus narrow and shallow at the beak, widening and deepening anteriorly 
to the middle of the valve, becoming less deep from there to the anterior margin. 
