Fauna of the Morrow Group 
205 
margin where a notable thickening of the shell may be observed 
in many individuals. Fine thread-like radiating striae may be 
observed in some specimens. 
Horizon and locality. Brentwood limestone: near Fayette- 
ville, Arkansas (Stations 135 and 152). 
Composita biplicata n. sp. 
Plate XIV, figures 5-5c. 
Description. Shell below medium size, sub-pentagonal in out- 
line, longer than wide, greatest width near middle, valves sub- 
equally convex, mesial fold and sinus developed anteriorly and 
bearing a strong plication along the middle line, anterior margin 
corrugated. The dimensions of the holotype are: length, 22.6 
mm. ; width, 19.7 mm. ; thickness, 11.5 mm. 
Pedicle valve most strongly convex in the umbonal region, the 
surface curving abruptly toward the cardinal margin and less 
strongly toward the lateral and anterior margins; mesial sinus 
originating near the middle of the valve and consisting of two 
rounded sinuses, slightly increasing in width and depth toward 
the margin, which border a fairly strong mesial fold, the inner 
margins of the sinuses more sharply defined than the lateral, 
and the convexity of their floors increasing anteriorly; beak 
somewhat tumid and incurved above that of opposite valve, 
truncated in a plane at an angle with the plane of union of the 
valves by a large sub-circular foramen. 
Brachial valve more strongly convex transversely than longi- 
tudinally; mesial sinus originating in front of the middle and 
bordered by two faintly outlined lateral ridges converging gently 
toward the umbo and defined by the stronger curvature of the 
anterio-lateral margins and the mesial fold; beak full but not 
prominent, incurved into delthyrium of pedicle valve. 
Surface of both valves marked by concentric lines of growth, 
in most cases elevated above shell surface and more numerous 
toward the anterior margin than on the umbo, crossed by very 
fine, thread-like, radiating striae. 
Horizon and locality. Morrow formation: near Ft. Gibson, 
Oklahoma (Station 301). 
