276 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 127 
the anterior slope. Lateral slopes concave, moderately steep. Foramen located 
about 3 mm. posterior to the apex. Slit formed by migration of foramen, about 
3 mm. long, narrow, but widening slightly near the foramen. 
Brachial valve forming a depressed cone but with the center or apex located 
about one-third the valve length from the posterior margin. Apex small, nearly 
smooth and bluntly rounded. All slopes to the margins gentle. 
Surface of both valves marked by concentric undulations which are more ele- 
vated and regular on the pedicle valve and more gently rounded, lower and 
crowded on the brachial valve and near the margins. 
Measurements in mm.—Holotype, length 18.0, width 17.0, height 5.5. 
Types.—Holotype: 109632f; figured paratypes: 109428a,b, 109632a,b,d,e, 
109634b, 117953a; unfigured paratypes: 109632c, 109633a-e, 109634a,c-f, 
117953b-j. 
Horizon and locality—Bromide formation (Pooleville member-O-roplecia 
gouldi zone) in Oklahoma: Rock Crossing of Hickory Creek, about center sec. 
35, 1.5 S.; R.1 E., Criner Hills, Carter County. 
Discussion.—This species is characterized by the biconical form and the nearly 
central position of the beak of the brachial valve. In this latter respect it differs 
strongly from all species of Schizotreta. Only one other species of Orbiculoidea 
was found in the formations represented in this monograph. It is not comparable 
to O. lamellosa (Hall) from the Trenton limestone, which is a much smaller 
species. 
ORBICULOIDEA LINVILLENSIS Cooper, new species 
Plate 20, B, figures 4, 5 
Small, longitudinally broadly elliptical in outline with the length slightly 
greater than the width. Sides slightly flattened, anterior and posterior rounded. 
Surface marked by fine concentric lines as usual in the genus. 
Pedicle valve with apex located at center; anterior and posterior slopes mod- 
erately convex, the posterior slope the steeper of the two. Lateral slopes flattened, 
moderately steep. Pedicle groove long and slender, not quite 2 mm. in length. 
Apex erect. 
Brachial valve in the form of an irregular cone, the apex located one-third the 
length from the posterior margin. Anterior slope steep and deeply concave ; pos- 
terior slope steep but nearly flat in profile. Lateral slopes flat but steep. Two 
muscle scars indistinctly visible anterolateral of the apex, and from these scars 
2 pallial trunks extend, swing together for the length of about I mm., then diverge 
slightly and extend nearly to the front margin. 
Measurements in mm.—Holotype, pedicle valve: length 9, width 8.2, apex 4.4 
anterior to posterior margin. Paratype, brachial valve (109637b): length 8.3, 
width 7.4, apex located 2.7 mm. anterior to the posterior margin. 
Types.—Holotype: 109637a; paratype: 109637b. 
Horizon and locality—Oranda formation in Virginia: 0.6 mile northwest of 
Linville Station, Broadway (15’) Quadrangle. 
Discussion.—This species is characterized by the central position of the pedicle 
