450 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 127 
the gently convex brachial valve. In size it approaches P. transversa but differs 
in having a less carinate pedicle valve and less transverse outline. It differs from 
P. tenuis in possessing much stronger costae. It is also suggestive of P. pennsyl- 
vanica but has a more convex pedicle valve. 
PLECTORTHIS MAZOURKAENSIS Phleger 
Plectorthis mazourkaensis PHuEcER, Bull. S. California Acad. Sci., vol. 32, pt. I, p. 9, pl. 2, 
figs. 3-5, 1933. 
Horizon and locality—Mazourka formation in California: Mazourka Canyon, 
about 4 mile below the Lead Canyon trail, Inyo Mountains. 
PLECTORTHIS OBESA Cooper, new species 
Plate 92, F, figures 34-38; plate 260, A, figures 1-5 
Shell large for the genus, subequally biconvex; hinge narrow; greatest width 
at the middle; sides and anterior margin rounded; anterior commissure recti- 
marginate ; surface provided with 27 narrowly rounded, elevated costae separated 
by spaces wider than the width of the costae. Each costa bearing a double row 
of pits on its surface. 
Pedicle valve moderately convex in lateral profile, the median region somewhat 
flattened but the anterior and posterior thirds convex. Anterior profile broadly 
and moderately convex. Umbonal region swollen; entire valve inflated but with 
steep posterolateral slopes. Interarea short, apsacline. 
Brachial valve moderately convex but with the posterior and anterior thirds 
somewhat abruptly curved. Anterior profile broadly convex with the median 
region somewhat flattened and the sides short and steep. Entire valve inflated ; 
sulcus originating on the umbo and extending to the anterior margin, shallow and 
narrow and occupied by 2 costae. All slopes to the margins precipitous. 
Measurements in mm.—Holotype, length 17.7, brachial length 16.9, width 19.2, 
hinge width 13.4, thickness I1.9. 
Types.—Holotype: 117020, figured paratype 124233a. 
Horizon and locality—Dark shale under Eureka quartzite, in Nevada: Ridge 
east of Martin Ranch; under 2 knobs of Eureka quartzite, north side canyon, 
3.1 miles N. 32° E. of Blair (Segura) Ranch, Antelope Mountains, Roberts 
Mountains (1°) Quadrangle. 
Discussion—This species suggests P. ponderosa, but it does not attain the 
large size of that species and is a thicker-shelled and much deeper species. 
PLECTORTHIS PATULA (Phleger) 
Plectorthis patulus Puuecer, Bull. S. California Acad. Sci., vol. 32, pt. 1, p. 10, pl. 2, figs. 1-2, 
1933. 
Horizon and locality—Mazourka formation in California: In Mazourka 
Canyon, about 4 mile below the Lead Canyon trail, Inyo Mountains. 
