176 
Kirtley F. Mather 
in front into the gently convex anterior margin; beak acute, 
extended conspicuously beyond that of brachial valve, and very 
slightly incurved ; delthyrium triangular, higher than wide, 
partially closed by a pseudo-deltidium, and with a circular fora- 
men ; surface bearing about nine simple angular plications orig- 
inating in front of the umbo and rapidly increasing in strength 
to the margins, the three mesial plications depressed below the 
one on either side which intersects the anterio-lateral extremity 
of the valve, those beyond decreasing in strength posteriorly. 
Brachial Valve as deep as, or deeper than, the pedicle, the 
greatest convexity a little posterior to the middle, the surface 
curving very gently at first but soon more rapidly toward the 
lateral margins and anterio-lateral extremities, and curving very 
slightly toward the anterior margin ; beak broad and umbo some- 
what flattened transversely, the beak closely incurved beneath 
that of opposite valve; the surface bearing about 10 plications 
entirely similar to those of pedicle valve, with usually 4 slightly 
elevated near the anterior margin to form a mesial fold which 
quickly becomes obsolete posteriorly. Internally the rostral cav- 
ity of the valve bears a strong mesial septum. 
The minute surface markings of both valves consist of fine, 
close-set concentric lines of growth which are about as con- 
spicuous in the umbonal region as near the margins. Shell 
structure fibrous. 
Remarks. This species is probably the immediate ancestor of 
Pugnoicles uta (Marcou) abundant in the mid-Pennsylvanian ; 
strata and is distinguished from that form by the smaller size, | 
the triangular outline, and the much smaller convexity of the 
valves which characterize the earlier type. ; 
Horizon and locality. Hale formation : East Mountain, Fay- | 
etteville, Arkansas (Station 137). Brentwood limestone: near 
Fayetteville, Arkansas (Stations 134 and 135). Morrow forma- 
tion: near Choteau, Oklahoma (Station 306). 
Genus RHYNCHOPORA King 
Rhynchopora magnicosta n. sp. 
Plate X, figures 11 -11c. 
Description. Shell below medium size, sub-pyramidal in form, 
sub-triangular in outline, broader than long, the greatest breadth 
