164 
Kirtley F. Mather 
17.3 mm. ; greatest width of valve, 23.3 mm. ; length of hinge-line, 
20.0 mm. ; convexity of pedicle valve, about 6.0 mm. 
Pedicle valve with low convexity, not produced far beyond 
visceral region in any of the two-score specimens at hand ; greats 
est convexity in the umbonal region posterior to the middle, the 
valve sloping rather abruptly toward the hinge-line and ears but 
quite gently toward the lateral and anterior margins ; beak small 
and incurved close to hinge-line ; umbonal region expanding rap- 
idly, not projected far beyond hinge-line; umbonal angle about 
130° ; ears not sharply defined from umbonal slopes, cardinal 
angle approximating 90° or very slightly obtuse; venter flat- 
tened, rounding regularly into the main flanks on either side, 
modified by a shallow sinus which originates in the umbonal 
region and is normally inconspicuous though varying in depth 
and sharpness of definition. Surface of valve bearing numerous, 
close-set, but comparatively strong, radiating costae which orig- 
inate on the umbo as very slender striae but increase regularly 
in width and strength toward the anterior and lateral margins 
without any very notable increase in number though bifurcation 
and interpolation of costae in some instances occurs; about 20 
costae occupy a space of 5 mm. on the umbo and about 10 the 
same distance near the anterior margin of the venter ; radiating 
costae are crossed by numerous concentric ribs which are about 
as strong on the venter as on the flanks although disappearing on 
the ears ; about 5 ribs occupy the space of 5 mm. on the venter 
while toward the beak they are more closely spaced; there is a 
strong tendency for the costae to develop nodes at the inter- 
sections with the ribs ; a few bases of erect tubular spines occur 
irregularly scattered over the surface of the valve and in the 
depressions which separate the ears from the cardinal slopes 
show a tendency to become arranged in two alternating linear 
series parallel to the anterior margins of the ears. 
Brachial valve unknown. 
Horizon and locality. Hale formation: East Mountain, Fay- 
etteville, Arkansas (Station 136). Brentwood limestone: near 
Fayetteville (Stations 134 and 150) and West Fork (Station 
154), Arkansas; Sawney Hollow, Oklahoma (Station 210). 
Kessler limestone: near Brentwood (Station 144) and on East 
Mountain, Fayetteville (Station 209), Arkansas. Morrow for- 
