342 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 127 
Horizon and locality —Pogonip group (mottled zone 700 feet below the Eureka 
quartzite) in Nevada: At the base of the first ridge east of Frenchman Flat, Las 
Vegas (1°) Quadrangle. 
TREMATORTHIS ROBUSTA Cooper, new species 
Plate 29, C, figures 21-27 
Shell subquadrate in outline with gently rounded sides and anterior margin; 
biconvex with the pedicle valve having the greater depth; hinge narrower than 
the shell width at the middle; surface costellate, 2 to 3 costellae in the space of 
I mm. at the anterior margin. 
Pedicle valve with long and strongly apsacline interarea; gently convex in 
lateral profile; anterior profile strongly convex and with long, gently convex 
slopes; umbo carinate, the narrow swelling extended anteriorly to the front 
margin as an ill-defined fold. Pseudodeltidium narrowly arched, short and with 
a minute foramen. 
Pedicle interior with large, thick teeth; dental plates nearly obsolete but ap- 
pearing as a ridge on the sides of the delthyrial cavity ; muscle area somewhat 
thickened ; delthyrial cavity deep; vascula media subparallel. 
Brachial valve marked medially by a broad and shallow sulcus extending from 
the beak to the anterior margin ; areas bounding the sulcus gently convex; lateral 
profile gently and evenly convex; anterior profile broadly sulcate; posterolateral 
areas small, with a gentle slope. 
Brachial interior with stout brachiophores, short median ridge and trilobed 
cardinal process. 
Measurements in mm.—Holotype, length 7.3, width 7.7, hinge width 6.6, 
thickness 3.8. 
Types.—Holotype: 116865a; figured paratype: 116865c; unfigured paratypes: 
116865b,d. 
Horizon and locality—Pogonip group (mottled zone 700 feet beneath the 
Eureka quartzite) in Nevada: At the base of the first ridge east of Frenchman 
Flat, Las Vegas (1°) Quadrangle. 
Discussion.—This species differs from T. masom by its robust form and the 
parallel arrangement of the vascula media in the pedicle valve. 
TREMATORTHIS TENUIS Cooper, new species 
Plate 29, B, figures 16-20 
Shell large for the genus, somewhat quadrate in outline; hinge forming the 
widest part; sides very gently convex ; cardinal extremities acute; anterior mar- 
gin strongly rounded. Surface fascicostellate, costellae forming bundles of 3. 
Pedicle valve subcarinate, a median costella the strongest on the valve origi- 
nating at the beak and extending to the anterior margin; gently convex in lateral 
profile but having a greater depth than the brachial valve ; anterior profile form- 
ing a low triangle with gently sloping lateral areas; beak small; interarea apsac- 
line. Pseudodeltidium narrowly arched, foramen minute. 
