204 
Kirtley F. Mather 
Brachial valve with greater convexity transversely than longi- 
tudinally; mesial fold obsolete, the middle portion of the valve 
truncated to fit the flexure of the pedicle valve, the anterio-lateral 
margins extended with slightly increased curvature to All in the 
angles beneath the lateral ridges of the opposite valve, anterior 
margin flattened or slighty crimped upward along line of contact 
between valves; beak not prominent, incurving beneath that of 
opposite valve. 
Surface of both valves marked by close-set concentric lines of 
growth crossed in many specimens by fine thread-like radiating 
striae. 
Horizon and locality. Brentwood limestone: near Fayette- 
ville, Arkansas (Stations 134 and 135). Morrow formation: 
near Ft. Gibson, Oklahoma (Station 301). 
Composita gibbosa n. sp. 
Plate XIII, figures 16 - 1 8 c. 
Description. Shell small, gibbose, sub-oval in outline, longer 
than wide, greatest width at the middle or slightly anterior 
thereto, an undefined fold and sinus developed anteriorly. The 
dimensions of four typical specimens are: length, 12.1 mm., 11.4 
mm., 11.2 mm., 9.5 mm. ; width, 10.2 mm., 10.4 mm., 9.5 mm., 
8.2 mm. ; thickness, 7.4 mm., 6.7 mm., 7.6 mm., 6.0 mm. 
Pedicle valve broadly ovate in outline, surface curving abruptly 
from umbo toward cardinal margin and more gently toward 
lateral and anterior margins, mesial sinus shallow, undefined, 
originating in front of the middle and extended anteriorly with 
an increasing convexity to meet the brachial valve, in most speci- 
mens a faint narrow sulcus originates on the umbo and con- 
tinues to the front occupying the bottom of the sinus; beak 
somewhat tumid, incurved above that of opposite valve, trun- 
cated by the sub-circular foramen. 
Brachial valve sub-circular in outline, strongly convex trans- 
versely and less strongly longitudinally; mesial fold obsolete or 
faintly developed anteriorly, anterio-lateral areas more strongly 
convex than the middle portion of valve; beak not prominent, 
incurved beneath that of pedicle valve. 
Surface of both valves covered by concentric lines of growth 
which ordinarily are strongly elevated, especially toward the 
