PART I CHAZYAN AND RELATED BRACHIOPODS—COOPER 729 
homa: From bed 22 of Decker west of U. S. Highway 77, NE# sec. 25, T. 2 S., 
R. 1 E., north of Ardmore, Carter County; in reef limestone on the west side 
of the cut on Oklahoma Highway 18, 1.8 miles south of Sulphur, Murray 
County ; sec. 20, T. 6 N., R. 15 W., north side of Wichita Mountains; 25 to 46 
feet above the Bromide sand on Spring Creek, sec. 17, T. 2 S., R. 1 W., Murray 
County ; West Spring Creek, east of Pooleville, Murray County; 150 feet below 
the top at the hairpin bend 0.1 mile below the Observation Point on the Scenic 
Drive, 1 mile east of U. S. Highway 77, center NW4INW3 sec. 29, T. 1 S., R. 2 
E., Murray County. 
Yellow shaly limestone just above a 25-foot bed of sandstone at base of the 
Eureka group in Nevada: In the saddle just north of hill 8167, Martins Ridge, 
Monitor Range, Roberts Mountains (1°) Quadrangle. 
Doubtfully referred to this species are specimens from the Sevier formation in 
Tennessee: In a ravine on Tennessee Highway 73, 0.3 mile south-southeast of 
Walker Chapel, Kinzel Springs (T.V.A. 148-NE) Quadrangle; on Tennessee 
Highway 71 (Chapman Highway) 1.58 miles east-northeast of Cusick, Walden 
Creek (T.V.A. 156-SW) Quadrangle. 
Discussion.—This species as here described is somewhat variable, but it is 
especially characterized by its transverse form, large size, and subrectangular out- 
line. It is most like S. triseptatus from which it can be distinguished by the 
generally wider shells with more nearly rectangular cardinal extremities in the 
young. Inasmuch as S. triseptatus and S. lamellosus are both variable species, 
some specimens of each resemble the other markedly. Narrow individuals occur 
in both species, but the prevailing tendency is for the Oklahoma and Nevada 
specimens to be wider than the Tennessee and Virginia ones. Sowerbyites 
lamellosus is generally a smaller species than S. gildersleevei and does not attain 
the elaborate development of the interior seen in that Virginia species. The cal- 
losities at the front of the diductors are much stronger in the Virginia shells 
than in the Oklahoma ones. 
Three lots of Sowerbyttes from the Sevier formation of East Tennessee have 
been placed tentatively in this species. The specimens are not well preserved as 
they occur in a fine-grained sandstone and their exteriors are exfoliated. They 
do, however, have the somewhat transversely rectangular form of the Oklahoma 
and Nevada specimens and seem to occupy a somewhat post-Lincolnshire 
horizon. 
SOWERBYITES SUBNASUTUS Cooper 
Plate 182, A, figures I-11 
Species of about medium size for the genus, semielliptical in outline. Hinge 
forming the greatest shell width; cardinal extremities alate. Anterior margin 
somewhat narrowly rounded; lateral margins sloping obliquely anteromedially. 
Surface ornamented by costellae of unequal size ; 9 of the larger costellae occupy 
a space of 5 mm. at the front of the holotype. Short concentric lamellae also 
present on the exterior. 
