PART I CHAZYAN AND RELATED BRACHIOPODS—COOPER 343 
Brachial valve very gently convex in lateral profile; anterior profile deeply 
indented medianly and with lateral areas gently swollen ; umbonal region sulcate, 
sulcus widening and deepening to the anterior margin where it is broad and 
deep; areas bounding sulcus forming low plications; posterolateral areas small, 
gently concave. Cardinal process a thin, delicate blade. 
Measurements in mm.—Holotype, length 7.0, width 8.6, hinge width 9.6, thick- 
ness 2.8. 
Types.—Holotype: 117978a; unfigured paratype: 117978b. 
Horizon and locality—Pogonip group (mottled zone 700 feet below the 
Eureka quartzite) in Nevada: At the base of the first ridge east of Frenchman 
Flat, Las Vegas (1°) Quadrangle. 
Discussion—This species suggests Orthidiella extensa in its slender profile 
and deeply sulcate brachial valve; it differs from Orthidiella carinata in its profile 
and the deep sulcation of the brachial valve. It differs from Trematorthis mason 
in being large, more strongly costellate, in having a minute foramen and in hav- 
ing a deeper sulcus. The species suggests Orthidiella, but that genus has no 
apical foramen. It is a more slender shell than T. robusta. 
Family HESPERORTHIDAE Cooper, new family 
Progressive Orthacea having long and wide palintropes, subreniform ovarian 
impressions, and subparallel vascula media. Brachiophores long and more ad- 
vanced than in the Orthinae. Remnants of a pseudodeltidium and achilidium are 
retained in some genera. Delthyrium also margined by lateral plates in some 
genera. 
This family is erected for the Orthacea with long, slender brachiophores which 
are commonly triangular in cross section. These contrast strongly with the 
short brachiophores of the Orthinae. Another feature of interest is the presence 
of a small pseudodeltidium which arches under the sides of the delthyrium at 
the apex of the shell. This is partially covered, in some specimens, by lateral 
plates that extend over the delthyrial edge. 
Subfamily HesprrorTHINAE Schuchert and Cooper, 1931 
Hesperorthidae with strong radial ornamentation on the exterior and without 
concentric lamellae. 
Genus HESPERORTHIS Schuchert and Cooper, 1931 
Hesperorthis ScHucuert and Cooper, Amer. Journ. Sci., 5th ser., vol. 22, p. 244, 1931; Mem. 
Peabody Mus. Nat. Hist., vol. 4, pt. 1, p. 85, 1932. 
Hesperorthis is distinguished from Orthambonites, to which it has great re- 
semblance, by certain external and internal features. On the outside Hesperorthis 
always has strongly unequal valves, the pedicle valve is always the deeper but 
the brachial valve is more often gently concave, more rarely slightly convex. 
The palintrope of the pedicle valve is always fairly long and is usually curved. 
