PART I CHAZYAN AND RELATED BRACHIOPODS—COOPER 447 
Horizon and locality.—Joins formation in Oklahoma: In SW} sec. 2, T.1 S., 
R. 1 W., about 4 miles east of Hennepin, Murray County; Falls Creek, Murray 
County ; + mile west of Highway 77, sec. 25, T. 2 S., R. 1 E., Carter County ; 
bed 3 of section, 1 mile west of Mill Creek, Johnston County. 
Discussion—This species differs from D. nevadensis, also from the Joins 
formation, in its stronger and more distant ribbing. Although D. costata has 
some intercalated ribs, the surface is never as closely covered as in D. nevadensis. 
DESMORTHIS NEVADENSIS Ulrich and Cooper 
Plate 50, H, figures 35-39; plate 83, A, figures I-15 
Orthis perveta Watcort (not Conrad), U. S. Geol. Surv. Mon. 8, p. 72, pl. 11, figs. 3, 3a, b, 
1884. 
Desmorthis nevadensis Utricn and Cooper, Geol. Soc. Amer. Special Pap. 13, p. 159, pl. 30A, 
figs. 1-16, 1938. 
Types.—Holotype: 91344a; paratypes: 91344b-k, 91342a-d; complete speci- 
men figured by Walcott as Orthis perveta: 17235; figured hypotype: 1108172 ; 
figured specimen: 110806a. 
Horizon and locality—Upper Pogonip group (Desmorthis zone) in Nevada: 
At the west end of Lone Mountain, 18 miles northwest of Eureka, Roberts 
Mountains (1°) Quadrangle. 
Joins formation in Oklahoma: SW43 sec. 2, T. 1 S., R. 1 W., 4 miles east of 
Hennepin, Murray County; Henryhouse Creek, Murray County; 4 mile west 
of U. S. Highway 77, sec. 25, T. 2 S., R. 1 E., Carter County. 
Genus PLECTORTHIS Hall and Clarke, 1892 
Plectorthis HALL and CLARKE, Pal. New York, vol. 8, pt. I, pp. 194, 221, 1892——SCHUCHERT 
and Cooper, Mem, Peabody Mus. Nat. Hist., vol. 4, pt. 1, p. 57, 1932. 
PLECTORTHIS AUSTRALIS Cooper, new species 
Plate 71, E, figures 21-26 
Shell of about medium size for the genus, subquadrate in outline, slightly 
wider than long; hinge narrow; greatest width at about the middle; sides and 
anterior margin gently rounded; anterior commissure rectimarginate. Surface 
marked by about 44 narrowly rounded, elevated costae intercalated in 2 genera- 
tions and an incipient one appearing at the front margin. 
Pedicle valve gently convex in lateral profile with the greatest convexity ap- 
pearing just anterior to the umbo; anterior broadly convex with the median 
region somewhat narrowly rounded ; umbo and median region somewhat narrowly 
swollen; anteromedian region and flanks moderately inflated; lateral slopes to 
margin long and moderately steep. Interarea short, curved, gently apsacline. 
Brachial valve gently convex in lateral profile with the maximum depth occur- 
ring at about the middle; anterior profile broadly and gently convex and with 
median region depressed slightly into a shallow sulcus. Umbo moderately swol- 
len; sulcus originating on the umbo, shallow, widening but not deepening to the 
