216 
Kirtley F. Mather 
and parallel, anterior margin sub-perpendicular to cardinal line, 
meeting it in a right angle but rounding into the ventral margin 
below; posterior margin oblique, joining the hinge-line at an 
obtuse angle and curving narrowly into the ventral margin ; beak 
inconspicuous, situated nearer the anterior extremity than the 
middle of the hinge-line; umbonal region somewhat flattened 
toward the middle of the valve; umbonal ridge rounded and 
poorly defined, the posterior portion of the valve above the ridge 
moderately flattened; surface of valve cancellated by fine, close- 
set, concentric and radiating striae, the radiate markings slightly 
the stronger over the posterior half of the shell while the con- 
centric lines are the stronger over the anterior portion. 
Dimensions of the type specimen, a right valve, are: width, 
7.4 mm. ; height, 4.0 mm. ; length of hinge-line, 5.6 mm. ; con- 
vexity of valve, about 1.5 mm. 
Horizon and locality. Brentwood limestone: near Fayette- 
ville, Arkansas (Station 135). Morrow formation: near Cho- 
teau, Oklahoma (Station 306). 
Genus CYPRICARDINIA Hall 
Cypricardinia laevicula n. sp. 
Plate XV, figure 20. 
Description. Shell small, very inequilateral, transversely sub- 
ellipsodial, about twice as wide as high, hinge-line about two- 
thirds as long as the greatest width below and rounding grace- 
fully into the oblique ventral margin at the posterior extremity, 
making the posterior portion of the valve moderately compressed 
vertically; anteriorly there is a distinct lobate extension of the 
shell in front of the umbonal elevation, and the ventral margin 
curves broadly upward into the convex anterior outline which 
becomes distinctly concave close to the beak; beak situated at 
the anterior extremity of the hinge-line, umbo tumid, surface 
of shell sloping somewhat abruptly toward the anterior and more 
gently toward the posterior extremities, convexity of valve 
greater near cardinal than near ventral margins; a faint con- 
striction originates on the umbo and passing obliquely across the 
valve meets the ventral margin a little in front of the middle; 
surface of shell ornamented by very faint, concentric undulations 
which may also be observed on the surface of the internal cast.. 
