PART I CHAZYAN AND RELATED BRACHIOPODS—-COOPER 589 
of subequal depth, the brachial valve being slightly the deeper ; anterior commis- 
sure very broadly uniplicate. Surface mostly smooth except for the anterior 
margin which is marked by 6 short costae. 
Pedicle valve evenly and moderately convex in lateral profile; broadly and 
gently inflated ; sides with a moderately deep groove marking off a marginal plica 
extending from the beak about four-fifths the distance to the margin. 
Brachial valve moderately convex in lateral profile; anterior profile moderately 
convex ; umbonal and median regions fairly strongly swollen ; sides precipitous. 
Measurements in mm.—Holotype, length 10.2, brachial length 10.0, midwidth 
8.2, hinge width 1.5, anterior width 9.5, thickness 5.7. 
Type.—Holotype: 117171. 
Horizon and locality—Pogonip group (sponge beds=Anomalorthis zone), 
in Nevada: In Ikes Canyon, # mile above the mouth, east side of the Toquima 
Range, Roberts Mountains (1°) Quadrangle. 
Discussion.—This species may be identified by its generally compressed form 
and the presence of a few strong costae confined to the anterior margin. The 
species is quite unlike J. costata Ulrich and Cooper which is more robust and 
which has the costae extending at least to the middle. It also differs from 
I. paucicostata which is more broadly triangular and in which the costae are 
stronger and occupy the anterior third. 
IDIOSTROPHIA PERFECTA Ulrich and Cooper 
Plate 109,A, figures 1-8 
Idiostrophia perfecta ULRicH and Cooper, Journ. Paleont., vol. 10, No. 7, p. 631, 1936; Geol. 
Soc. Amer. Special Pap. 13, p. 249, pl. 56F, figs. 15-17, 19-21, text figs. 12, 13, 1938. 
Types.—Holotype: 92885a; paratypes: 92885b-d; figured hypotype: 117173. 
Horizon and locality—Mystic Conglomerate, in Quebec, Canada: Range 6, 
Lot 20, Stanbridge Township. 
Table Head series (middle to upper half) in Newfoundland: 24 to 3 miles 
south of Belle Burns. 
IDIOSTROPHIA PLICATA Cooper, new species 
Plate 109, C, figures 14-17 
Shell small for the genus, triangular in outline ; length and width nearly equal ; 
sides straight, forming an apical angle of about 70°. Anterior margin broadly 
rounded. Anterior commissure broadly uniplicate (?). Surface of posterior half 
smooth, anterior half marked by 4 strong, low, rounded costae. 
Pedicle valve moderately convex in lateral profile, broadly convex in anterior 
profile. Umbonal marginal plica narrow ending anteriorly in a narrowly rounded 
projection. Lateral area narrowly grooved and concave. Beak small, incurved 
slightly. 
Brachial valve somewhat more convex and somewhat deeper than the pedicle 
valve in lateral profile ; anterior profile broadly and gently convex. Umbo nar- 
rowly swollen; median region gently inflated. Lateral area broadly concave. 
