354 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 127 
profile broadly convex. Umbonal region convex ; median portion of valve slightly 
swollen. Anterior and lateral slopes gently convex, the lateral slopes the steeper. 
Interarea strongly apsacline, nearly flat; delthyrium without a pseudodeltidium 
as far as can be observed. Interior with short, receding dental plates; muscle 
area as usual in the genus. 
Brachial valve with posterior half very slightly convex but the anterior half 
perceptibly concave; median region slightly depressed by a shallow and wide 
sulcus originating at the umbo. Flanks bounding sulcus very slightly convex; 
cardinal extremities slightly deflected toward the pedicle valve. Interarea short, 
anacline. Interior with short but stout brachiophores which are triangular in sec- 
tion because they have a keel along the outer face. Notothyrial platform not 
greatly thickened; cardinal process slender but the thickened base bearing a 
longitudinal slit. 
Measurements in mm.— 
Hinge 
Length Width width 
Holotype (pedicle valve).................. 12.5 15.5 15.2 
Paratype (brachial valve 109975a)......... 9.7 12.0 13.5 
Types.—Holotype: 109975b; figured paratypes: 109975a,c,e; unfigured para- 
types: 109975d,f-1. 
Horizon and locality—Tulip Creek formation in Oklahoma: West Spring 
Creek, northeast corner sec. 1, T. 2 S., R. 2 W., 3 miles east of Pooleville ; along 
an east-west road 2 miles southwest of Nebo, Murray County. 
Discussion.—The closest relative of this species is H. ignicula (Raymond) 
from the Valcour limestone of the Chazy group of New York. In the Oklahoma 
species the costae are wider and separated by narrower striae, the pedicle beak 
is less incurved, and the same valve is less convex; inside the brachial valve the 
notothyrial platform is smaller and more thickened and the cardinal process 
stouter. 
HESPERORTHIS MULTICOSTATA Cooper, new species 
Plate 51, C, figures 14-18 
About medium size for the genus, slightly wider than long. Hinge about equal 
in width to the midwidth of the valves. Lateral margins nearly straight; antero- 
lateral margins gently curved; anterior margin somewhat narrowly rounded. 
Anterior commissure gently sulcate. Apical angle 130°. Surface ornamented by 
38 narrowly rounded costae separated by spaces narrower than the costae. 
Pedicle valve unevenly convex in profile; the greatest convexity located near 
the middle. Umbonal region flattened in lateral profile. Anterior profile nar- 
rowly convex with the flanks forming an angle of 110°. Umbonal region sub- 
carinate, the folding prominent at the middle but only slightly defined at the 
front. Lateral slopes steep and flat. Interarea strongly apsacline, forming an 
angle with the lateral commissure slightly more than a right angle; very slightly 
curved. Beak slightly incurved. 
Brachial valve gently convex in lateral profile; strongly indented medially in 
