PART I CHAZYAN AND RELATED BRACHIOPODS—COOPER 345 
Horizon and locality—Dark shales below the Eureka quartzite in Nevada: 
Below two knobs of Eureka quartzite, north side of canyon 3.1 miles N. 32° E. 
of Blair (Segura) Ranch, Antelope Mountains, Roberts Mountains (1°) Quad- 
rangle. 
Discussion.—This species is characterized by its straight sides, long interarea, 
deep pedicle valve, and strongly incurved beak. It is most like H. colei, but 
differs in having a longer and more curved interarea, deeper and more carinate 
pedicle valve, and a more broadly sulcate brachial valve. Hesperorthis virginica 
is not so deep as H. antelopensis, its interarea is more strongly apsacline, and 
its brachial valve is less sulcate. Hesperothis sulcata of Oklahoma in not so deep 
as the Nevada species, and its brachial valve is more deeply sulcate, so much so 
that the anterior commissure is definitely and strongly sulcate. 
HESPERORTHIS AUSTRALIS Cooper, new species 
Plate 49, C, figures 11-15; plate 53, A, figures 1-31 
Shell of medium size to small for the genus, slightly wider than long with the 
hinge forming the widest part or usually slightly narrower. Maximum shell 
width often just anterior to the hinge. Cardinal extremities varying from slightly 
obtuse to slightly acute. Lateral margins nearly straight and slightly oblique to 
very gently rounded. Anterior margin broadly rounded; the outline semiellip- 
tical. Surface costate, costae direct, narrowly rounded, and numbering 26 to 30. 
Interspaces occupied by several fine costellae, one of which may be stronger than 
the others, and by fine concentric fila. Costae seldom intercalated. 
Pedicle valve with lateral profile unequally convex and with the anterior flat- 
tened ; maximum convexity located just anterior to umbo. Anterior profile sub- 
carinate with the sloping sides forming an angle slightly less than 110°. Umbonal 
region somewhat swollen with the swollen portion extending to the front margin 
as a low fold. Lateral slopes long and fairly steep. Anterior slope long but less 
steep than the lateral ones. Beak slightly incurved and forming an angle of 
about 130° to 140°. Interarea strongly apsacline, slightly curved. Delthyrium 
long and narrow ; pseudodeltidium small. 
Brachial valve with lateral profile varying from very slightly concave to dis- 
tinctly but slightly convex ; anterior profile the same. Median sulcus originating 
at the umbo, expanding rapidly anteriorly to more than half the width; flanks 
bounding sulcus very slightly convex; posterolateral extremities nearly flat. 
Interarea short. 
Pedicle interior with deep, elongate delthyrial cavity bounded by strong reced- 
ing dental plates. Teeth small with deep fossettes. Vascula media not strongly 
developed. Brachial interior with long brachiophores and stout cardinal process 
often supplemented by lateral bosses. Median ridge low and stout, reaching 
anterior to the middle. 
