PART I CHAZYAN AND RELATED BRACHIOPODS—-COOPER 751 
Interarea long, gently apsacline. Interior with deep delthyrial cavity having 
thickened walls ; dental plates obscured by adventitious shell. Muscle field large, 
with elongated diductor scars occupying the posterior third ; vascula media short, 
separated by a narrow granular area. 
Brachial valve moderately deep, the depth increasing with age; maximum con- 
cavity just anterior to the middle; umbo and surrounding area gently concave; 
cardinal extremities flattened. Interarea short, hypercline. Interior with low 
visceral disk having a highly elevated median septum. 
Measurements in mm.— Brachial Surface Mid- Hinge —Thick- 
Length length length width width ness Height 
FIOMOGy PE sphaceet «9. - 3 10.4 8.8 13.5 14.2 TAANE uth 2:6 4.8 
Paratype (117438a)... 9.1 PEE 115 T2204. EADS ue 3.8 
: (117438b)... 80 6.9 10.5 10.0 Ter 22 3.4 
Types.—Holotype: 117438c; figured paratypes: 117438a,b,d,e. 
Horizon and locality—Dark shale below Eureka quartzite in Nevada: On 
the north-facing nose of hill 8167, Martins Ridge, Monitor Range, Roberts 
Mountains (1°) Quadrangle. 
Discussion.—Specimens of this species have a ragged appearance because of 
partial peeling of the fine lamellae. Transverse form, low convexity and a large 
pedicle muscle area characterize this species. Leptellina abbreviata is thought 
to be of about the same age, but L. incompta is a much less convex form. Leptel- 
lina sublamellosa is also characterized by prominent development of the lamellae, 
but the species is somewhat smaller, more transverse, and proportionately more 
convex. 
LEPTELLINA OCCIDENTALIS Ulrich and Cooper 
Plate 180, C, figures 30-37 
Leptellina occidentalis Utricu and Cooper, Geol. Soc. Amer. Special Pap. 13, p. 191, pl. 39B, 
figs. 3, 7, 1938. 
Additional material of this species collected after the appearance of Special 
Paper 13 has yielded specimens quite suitable for illustration of many of the 
characters mentioned in the original description. Not seen in the original ma- 
terial are the fine lamellae which cover most of the surface, like those of other 
well-preserved specimens of Leptellina from higher formations. This species is 
readily distinguished from all other known species of the genus by its small size, 
gently convex to nearly flat brachial valve, and the low and short visceral disk. 
Types.—Holotype: 92869a; paratypes: 92868a, g286gb-d, 92870a,b; figured 
hypotypes: 108133a-e, 117440a-c. 
Horizon and locality—Common in the topmost beds of the Pogonip group in 
Nevada: In Ikes Canyon, on the east side of the Toquima Range, Roberts 
Mountains (1°) Quadrangle. 
LEPTELLINA PLATYS Cooper, new species 
Plate 186, D, figures 13, 14 
Shell of about medium size for the genus, of low convexity, wider than long 
and with the hinge about equal to the maximum width. Sides slightly oblique, 
