588 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 127 
brachial fold. The uniplication of these shells is thus obscure but definite never- 
theless. 
IDIOSTROPHIA COSTATA Ulrich and Cooper 
Plate 100, B, figures 9-13 
Idiostrophia costata Utricu and Cooper, Geol. Soc. Amer. Special Pap. 13, p. 250, pl. 56C, 
figs. 4-6, 9-11, 1938. 
Type.—Holotype : 92886. 
Horizon and locality—Boulder in Mystic Conglomerate in Quebec, Canada: 
Range 6, Lot 20, Stanbridge Township. 
IDIOSTROPHIA NUDA Cooper, new species 
Plate 100, E, figures 21-29 
Shell of about medium size for the genus, length and width nearly equal, out- 
line triangular ; sides obliquely straight ; anterior margin broadly rounded; valves 
unequally convex, the brachial valve having the greater depth; anterior commis- 
sure broadly uniplicate ; surface of posterior four-fifths smooth; anterior margin 
marked by about 12 costae. 
Pedicle valve moderately convex in lateral profile; broadly convex in anterior 
profile; umbonal region swollen; median region inflated but not so strongly in- 
flated as the umbo; lateral region with a long, shallow depression marking a 
deflected area which defines a ridge extending from beak to slightly anterior to 
middle ; side grooved between ridge and lateral commissure. Tongue wide. 
Brachial valve unevenly convex in lateral profile with the umbonal region 
swollen and extending to the level of the beak of the pedicle valve; anterior com- 
missure narrowly convex; median region swollen; sides precipitous, with a 
groove parallel to the lateral commissure ; anterior forming a broad fold occupy- 
ing the entire width; anterolateral extremities produced into a short flange that 
fits into a slot at the posterior of the tongue of the pedicle valve. 
Interior unknown. 
Measurements in mm.—Holotype, pedical length 10.4, brachial length 10.3, 
midwidth 9.1, anterior width 10.8, thickness 7.8. 
Type.—Holotype: 117170. 
Horizon and locahty.—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. 
Discussion.—This species is characterized by its fairly strongly inflated valves 
and the lack of ornamentation on the posterior four-fifths. The costellae along 
the front margin are also fairly delicate. The species is thus unlike any of those 
described herein. 
IDIOSTROPHIA PAUCICOSTATA Cooper, new species 
Plate 109, D, figures 18-20 
Shell of about medium size for the genus, triangular in outline; sides nearly 
straight forming an angle of about 65° ; anterior margin broadly rounded. Valves 
