PART I CHAZYAN AND RELATED BRACHIOPODS—COOPER 299 
slightly less. Lateral margins gently rounded ; anterior margin broadly rounded. 
Anterior commissure rectimarginate. Surface marked by 30 to 32 strong, broadly 
rounded costae having interspaces about the same width as the costae. Inter- 
spaces marked by fine radial costellae and strong concentric fila, the latter best 
developed on the median and posterior parts but the radial costellae best shown 
along the anterior parts. 
Pedicle valve moderately convex in lateral profile with the most convex por- 
tion located in the posterior half, front half flattened. Anterior profile with flat- 
tened sides and narrowly rounded crest at the middle ; beak prominent forming an 
angle of 153°. Median region moderately swollen with moderately steep slopes to 
the posterolateral extremities, the slopes becoming less steep anteriorly ; anterior 
slope long and gentle. Interarea moderately long, apsacline. Muscle area small 
and narrow for such a large valve occupying less than a third the length. Dental 
plates short and receding; pallial impression indistinct. 
Brachial valve slightly convex in anterior and lateral profile; sulcus originat- 
ing at the posterior margin, narrow and shallow and extending to about the 
middle of the valve where it is lost in the general convexity of the surface. Flanks 
slightly swollen; cardinal extremities flattened. Interior with narrow and short 
but moderately deep notothyrial cavity ; cardinal process a thin septum ; brachio- 
phores long and slender; median ridge stout, reaching to the middle. Pallial 
impressions indistinct. 
Measurements in mm.— 
Hinge 
Length Width width Thickness 
Holotype (pedicle valve) ...... 20.7 24.5 20.7 57 
Raratype: GiOoS35p)) Gisec.e-< + 16.2 20.0 10.5 2.0 
Types.—Holotype: 109835c; figured paratypes: 109835a,b,d,f ; unfigured para- 
type: 109835e. 
Horizon and locality—Effna-Rich Valley formations in Virginia: At Porter- 
field Quarry, 5 miles east of Saltville, Maccrady (T.V.A. 218-NW) Quad- 
rangle. 
Botetourt formation in Virginia: Opposite road intersection with Hays 
Creek, 2 miles northwest of Brownsburg, Lexington (15’) Quadrangle; road to 
Smyth Gap, 3 miles southwest of Clinchburg, Glade Spring (T.V.A. 212-SE) 
Quadrangle. 
Discussion.—This species can be recognized by its large size and the strong 
concentric ornamentation between the costae. Specimens from the locality 2 miles 
northwest of Brownsburg measure over 30 mm. in width and about 25 mm. in 
length. This is one of the largest members of the genus so far found in this 
country. This species has some features of Hesperorthis as, for example, the 
long slender brachiophores and the transverse form of the pedicle valve. The 
few specimens showing the delthyrium have no trace of a pseudodeltidium but 
traces of lateral (deltidial?) plates are present. The size and ornamentation will 
distinguish this species from all others herein described (see O. neumant). 
