218 
Kirtley F. Mather 
length of shell, left valve convex, right valve concave. Dimen- 
sions of the holotype are: length, 12.5 mm.; height, 13.2 mm.; 
convexity of left valve, about 3.2 mm. ; length of hinge-line, 
6.0 mm. 
Left valve moderately inflated, convexity strong in umbonal 
region and near the ventral margin, more gentle throughout 
the central portion of the valve; beak prominent, narrow, and 
directed almost vertically ; auriculations comparatively small and 
poorly deflned, hinge-line extended about twice as far anteriorly 
as posteriorly, outline of anterior ear somewhat geniculate ; sur- 
face of shell ornamented by radiating striae and marked by con- 
centric lines of growth, striae obsolete over umbonal region, 
originating on the ears and a little above the middle of the valve 
as fine, somewhat wavy threads elevated above a flat floor and 
continuing to the margins, increasing slightly in strength and 
with a few new threads arising by implantation; at intervals 
near the margins appearing somewhat nodose; lines of growth 
consisting of slight, irregular elevations about as prominent in 
the umbonal region as below. 
Right valve gently and regularly concave with ornamentation 
probably similar to that of the left valve. 
Remarks. This species is based upon two specimens from the 
Brentwood limestone, one of which is nearly complete. The 
right valve is missing in one specimen and somewhat obscured 
in the other but the generic position of the form is unmistakable. 
The peculiar ornamentation, with the umbo devoid of sculpture, 
and the small size of the shell — the type specimen is an adult — 
serve to readily distinguish it from the other Carboniferous 
members of the genus. 
Horizon and locality. Brentwood limestone: northeast of 
Fayetteville, Arkansas (Station 135). 
Pseudomonotis inflata n. sp. 
Plate XV, figures 15, 15a. 
Description. Shell large, irregularly sub-circular in outline, 
length and height sub-equal, greatest length along a line which 
converges notably toward the hinge-line anteriorly, hinge about 
half as long as length below, left valve strongly inflated. Dimen- 
sions of the holotype are : greatest length, 35.7 mm. ; height 
