PART I CHAZYAN AND RELATED BRACHIOPODS—COOPER WAY 
ISOPHRAGMA PONDEROSUM Cooper, new species 
Plate 210, H, figures 25-32 
Large for the genus, subrectangular in outline; sides rounded, somewhat 
oblique; anterior margin broadly rounded; cardinal extremities acute; anterior 
commissure rectimarginate. Costellae strong, numbering 13 to 14 in 5 mm. at 
the front margin. 
Pedicle valve nearly flat in lateral and anterior profile; beak low; umbo nar- 
rowly rounded and extended to about the middle as a low fold; anterior half 
slightly depressed to form a barely perceptible sulcus ; sides bounding fold slightly 
concave on each side of the fold but becoming flat on the lateral extremities. 
Interarea moderately long, strongly apsacline; deltidium narrowly rounded. 
Brachial valve faintly convex in lateral and anterior profiles; umbo sulcate, 
the sulcus extending to about the middle; anterior third gently swollen to form 
a barely perceptible fold. Flanks bounding sulcus gently convex. Interior with 
septa united into a broad ridge rising to a crest at its free extremity slightly 
anterior to the valve middle. 
Measurements in mm.— 
Brachial Hinge 
Length length Midwidth width Thickness 
Piglotvine bs o).ca «\aeeeyd <in:c\e 0.0 Bemseis 10.7 9.9 13.6 ? 14.6-++ 2.9 
Paratype'r (1232012... 3 s<c00 ss 0.4 ? 14.8 173 ? 
a (8232698)  si6i.'ss Wa's.00 11.6 ? 15.8 16.4-++ ry 
Types.—Holotype: 123259a; figured paratypes: 123261, 123262a; unfigured 
paratypes: 123259b, 123262b. 
Horizon and locality. Y ellow-weathering limestone on 25-foot sandstone at 
base of Eureka group in Nevada: 2 and 3 miles north of Martin Ranch, Roberts 
Mountains (1°) Quadrangle. 
Discussion.—This is the largest species of the genus known. Like most of the 
other species the young stages are very transverse, but in this western form the 
cardinal angles tend to become less obtuse as adulthood and old age are reached. 
This species is suggestive of I. biseptatum but differs, besides its size, in being 
more elongate, having a flatter brachial valve and a more apsacline interarea. 
The Nevada species is much less transverse than J. ricevillense, is not so strongly 
sulcate, has a less pronounced brachial fold and a more apsacline interarea. 
ISOPHRAGMA RICEVILLENSE Cooper, new species 
Plate 170, A, figures I-15 
Shell large for the genus, wider than long and with the hinge forming the 
widest part of the shell ; cardinal extremities varying from a right angle to acute; 
anterior margin broadly rounded. Valves subequal in depth, the brachial valve 
having a slightly greater depth than the pedicle one. Surface marked by subequal 
narrowly rounded costellae, about 14 in the space of 5 mm. at the front of a valve 
7mm. long. Two generations of costellae intercalated between the primary ones. 
Pedicle valve flat or slightly concave in lateral profile; umbo slightly swollen ; 
