196 
Kirtley F. Mather 
latter case the plications are sub-angular ridges separated by 
depressions which are the reverse of the ridges but in the more 
common exfoliated condition the ridges appear to arise from 
a flat floor and to be separated by depressions much broader 
than the ridges themselves. In some instances, in addition, the 
intercostal depressions seem to be bounded by very faint second- 
ary ridges at either side. 
In mature individuals the great thickness of the valves and 
the flattening of the vertical lateral slopes are very conspicuous. 
Horizon and locality. Hale formation: East Mountain, Fay- 
etteville, Arkansas (Station 136). Brentwood limestone: near 
Fayetteville, Arkansas (Stations 134 and 135) ; Sawney Hollow, 
Oklahoma (Station 210). Morrow formation: near Ft. Gibson 
(Station 301), Choteau (Stations 302 and 307), and Gore (Sta- 
tion 305), Oklahoma. 
Hustedia miseri n. sp. 
Plate XIII, figures Jf.-6c. 
Description. Shell small, sub-ovate in outline, longer than 
wide, greatest width near mid-length of valves, valves sub- 
equally convex. The dimensions of three perfect specimens are : 
length, 8.0 mm., 7.0 mm., and 6.0 mm. ; width, 6.9 mm., 5.2 mm., 
and 5.4 mm. ; thickness, 5.0 mm., 4.5 mm., and 3.9 mm. 
Pedicle valve convex, the greatest convexity occurring about 
at the mid-length of the valve from which point the surface 
curves quite strongly toward the posterio-lateral margins and 
more gently toward the anterio-lateral and anterior margins; 
beak full, incurved above that of opposite valve, truncated by 
a small circular foramen; anterior margin very slightly trun- 
cated in the middle or even a little emarginated in some indi- 
viduals, curving broadly into the convex lateral margins on 
either side; surface bearing about 20 strong, simple, sub-angu- 
lar, radiating plications of about equal strength throughout the 
mesial portion of the valve but fainter towards the posterior 
lateral margins; a very faint undefined mesial sinus developing 
anteriorly in some individuals and causing the truncation or 
emargination of the anterior margin. 
Brachial valve nearly or quite as deep as the pedicle and of 
similar curvature and outline; the beak incurved beneath that 
