198 
Kirtley F. Mather 
perfectly with the Chester individuals in outline and shape. 
About 42 simple plications are present on each valve. 
The dimensions of the figured specimen are : length, 10.0 mm. ; 
breadth, 8.2 mm. ; thickness, 5.4 mm. 
Horizon and locality. Brentwood limestone: vicinity of Fay- 
etteville, Arkansas (Stations 148, 152, and 153). 
ATHYRID^ 
Genus COMPOSITA Brown 
Composita ozarkana n. sp. 
Plate XIII y figures 11-1 5c. 
Description. Shell typically medium though in some instances 
small in size; sub-ovate in outline, the larger specimens not un- 
commonly becoming subquadrate or subpentagonal, the width 
ordinarily as great as or slightly greater than the length but 
in some individuals less than the length, the greatest width 
normally slightly anterior to the mid-length of the shell but 
rarely slightly posterior thereto, fold and sinus developed an- 
teriorly. The dimensions of three specimens, the largest of 
which is the one selected as the holotype, are : length, 20.8 mm., 
16.4 mm., 10.7 mm. ; breadth, 21.0 mm., 16.2 mm., 10.8 mm., 
thickness, 13.8 mm., 10.6 mm., 6.7 mm. 
Pedicle valve most convex in and near the umbonal region, 
the surface curving abruptly toward the cardinal margin and 
more gently toward the lateral and anterior margins, the con- 
vexity decreasing anteriorly from the umbo until close to the ' 
margin where it may be somewhat increased over a short space ; 
the mesial sinus originating on the umbo at a point one-sixth 
to one-fifth the distance from beak to anterior margin as a 
faint narrow groove which deepens anteriorly and becomes a 
sulcus, ordinarily fairly well-defined, in the bottom of the broad, 
ill-defined, shallow sinus, which toward the anterior margin i 
occupies approximately one-fourth the width of the valve, with 
poorly defined lateral margins; in some specimens faint ridges 
separate the mesial depression from the lateral portions of the 
shell but these may be observed only at and near the anterior 
margins, as they do not originate until beyond the umbonal 
region ; beak strongly incurved, truncated obliquely to the plane 
