PART I CHAZYAN AND RELATED BRACHIOPODS—COOPER 309 
and steeply sloping sides. Median region inflated ; anterior slope long and steep. 
Posterolateral areas somewhat swollen and without steep slopes. Posterior mar- 
gins forming an angle of 120° with the beak. Interarea moderately long, slightly 
curved, apsacline. Delthyrium narrow, open. 
Brachial valve gently and evenly convex in anterior and lateral profiles. Sul- 
cus shallow, narrow, originating at the umbo and extending to the front margin 
where it occupies less than a quarter of the width. Sulcus occupied by 4 costae. 
Flanks gently convex. Umbonal region slightly swollen and with short but steep 
umbonal slopes. Posterolateral areas slightly flattened. Interarea very short. 
Measurements in mm.—Holotype, length 15.5-+, brachial length 13.0, width 
18.0, hinge width 16.0, thickness 8.6. 
Types.—Holotype: 109807a ; unfigured paratype: 109807b. 
Horizon and locality—Upper Pogonip group (Rhysostrophia zone) in Ne- 
vada: North side Ikes Canyon about 1 mile above the entrance, Toquima Range, 
Roberts Mountains (1°) Quadrangle. 
Discussion—This species is larger and more numerously costated than 
O. minusculus and is distinguished from O. michaelis by its smaller, narrower, 
and more distantly spaced costae, and the somewhat more transverse outline. 
ORTHAMBONITES PARVICRASSICOSTATUS Cooper, new species 
Plate 35, B, figures 11-25 
Shell small, slightly wider than long, lateral margins gently rounded to nearly 
straight, anterior margin narrowly rounded. Cardinal extremities obtuse or pro- 
duced into minute ears. Anterior commissure sulcate. Hinge slightly wider or 
slightly narrower than the shell width at the middle. Surface marked by 16 to 
18 angular costae that widen anteriorly. Interspaces about equal to the width of 
the costae, crossed by fine concentric fila. 
Pedicle valve with lateral profile gently convex and with the greatest convexity 
at about the middle. Umbo strongly convex. Anterior profile strongly humped 
in the median region with the flanks descending moderately steeply to the lateral 
margins. Median region of 5 largest costae somewhat elevated to produce a fold. 
Interarea short, beak strongly incurved. 
Brachial valve gently convex with the greatest convexity in the region posterior 
to the middle. Beak small, umbo small, smooth, short. Sulcus originating just an- 
terior to the umbo, widening rapidly to occupy about half the width. Four costae 
occupy the sulcus. Flanks defined by a slight elevation of the 2 costae bounding 
the sulcus. Posterolateral extremities flattened. 
Pedicle interior with small teeth, and short, stout dental plates. Muscle area 
slightly thickened, extending anterior to the anterior ends of the dental plates. 
Brachial interior with short, stout brachiophores supported by a moderately 
thickened notothyrial platform. Cardinal process thickened on the anterior face. 
