PART «I CHAZYAN AND RELATED BRACHIOPODS—COOPER 745 
Discussion.—The interior of this genus is most like that of Leptellina but dif- 
fers in not having a strong visceral disk marked by a shelf overhanging the 
geniculated portion of the brachial valve interior. It also differs from Leptellina 
in the development of the cardinal process. This structure in Leptellina consists 
of 2 lateral plates attached to the inner edges of the brachiophores which are 
fused to a ridgelike cardinal process; the latter may or may not overhang the 
notothyrial region to some extent. In Hesperomena, on the other hand, the 
cardinal process is constructed like that of Leptellina, but the fused structure is 
oval in outline and overhangs the notothyrial area to a considerable extent. 
The structure of the pedicle valve is also different from that of Leptellina. 
The latter genus has well-marked dental plates and a short, usually wide, muscle 
area. In Hesperomena, on the other hand, dental plates are wanting and the 
muscle area is large, with long diductor scars completely unlike those of Leptel- 
lina. Hesperomena in its lack of an overhanging visceral disk might be supposed 
to be primitive, but the cardinal process appears to be more advanced than in 
Lepiellina itself. 
HESPEROMENA LEPTELLINOIDEA Cooper, new species 
Plate 185, A, figures 1-10 
Shell moderately large, wider than long; hinge not equal to the greatest shell 
width which is near the middle; sides moderately rounded; anterior margin 
strongly rounded ; surface marked by distant rounded costellae. 
Pedicle valve unevenly convex in lateral profile, most convex just anterior to 
the middle where the valve is abruptly geniculate toward the pedicle valve; an- 
terior profile broadly and gently convex; posterior half gently convex; umbo 
narrowly swollen, the swelling forming a short and inconspicuous fold. Anterior 
half gently swollen and bent toward the pedicle valve to form a long trail with 
steep and long anterior slope. Interarea moderately long, apsacline. 
Brachial valve most concave just anterior to the middle; posterior half gently 
concave; anterior half deflected toward the brachial valve and forming steep 
slopes from the cardinal extremities to the anterior: umbo sulcate, the sulcation 
forming a shallow and narrow depression which is lost on the brachial trail. 
Interarea short, hypercline. 
Measurements in mm.—Holotype, length 14.7, brachial length 12.6, midwidth 
23.2, hinge width 19.2, thickness 3.4, height 5.9. 
Type.—Holotype: 117560. 
Horizon and locality—Pogonip group (upper 30 feet) in Nevada: About 
1 mile above the entrance to Ikes Canyon, Toquima Range, Roberts Mountains 
(1°) Quadrangle. 
Discussion.—No other species of this genus is known to which this one may be 
compared. The measurements of midwidth and hinge width given above are 
based on the half width of the shell. 
