PART I CHAZYAN AND RELATED BRACHIOPODS—COOPER 523 
Discussion.—Atelelasma perfectum occurs in abundance in the beds contain- 
ing Christiania at Friendsville and vicinity. The species is characterized by its 
large size, the moderately convex pedicle valve, and the gently convex, rather 
than flat, brachial valve. At Friendsville specimens are nicely silicified and can 
be developed for interiors. 
The species is also characterized by its somewhat square form which distin- 
guishes it from A. decorticatum and A. obscurum. 
ATELELASMA PLANUM Cooper, new species 
Plate 80, D, figures 20-24 
Shell small, plano- to concavo-convex ; hemipyramidal in outline. Hinge slightly 
narrower than the greatest width. Cardinal extremities auriculate. Lateral mar- 
gins nearly straight or sloping medially ; anterior margin broadly rounded. An- 
terior commissure slightly sulcate. Surface multicostellate ; costellae appearing 
in 4 generations by implantation. First generation forming strongest costellae 
which stand above the others ; subsequent costellae tending to form fascicles with 
2 or 3 generations in a fascicle. 
Pedicle valve hemipyramidal; very gently convex in profile. Slopes to lateral 
and anterior margins steep; interarea long, procline. Delthyrium wide. 
Brachial valve flat or gently concave, with a deep median sulcus originating at 
the umbo and extending to the front margin where it occupies about one-third 
the width. Flanks bounding sulcus slightly convex; anterolateral extremities 
slightly depressed. 
Measurements in mm.— 
Brachial Hinge 
Length length Width width Thickness 
Holotype . .calseca. wets 9.8 9.4 12.0 13.4 6.0 
Paratype (110142a) .. 9.3 0.4 13.5 13.4 4.9 
Types.—Holotype: 110142b; figured paratype: 110142a. 
Horizon and locality—Benbolt formation (?) in Tennessee: 14 miles south- 
east of Knoxville, Knoxville (T.V.A. 147-NW) Quadrangle. 
Discussion.—This species is characterized by its small size and the nearly flat 
brachial valve. In these respects it is like A.? multicostum and A. platys, but 
it differs from both of them in details of its ornamentation. The costellae of 
the former species are fine and even, whereas those of A. plana are differentiated 
into coarser and finer ones. The ornamentation of A. platys is characterized 
by spinelike projections along the costellae like those of A. oklahomense, but 
they are absent from 4. plana. 
ATELELASMA PLATYS Cooper, new species 
Plate 50, G, figures 29-34 
Shell small for the genus, wider than long with the hinge forming the widest 
part; sides sloping toward the middle; anterior margin broadly rounded ; an- 
