PLATH LXXXVI. 289 
BILLINGSELLA sTRIATA Walcott (p. 764). 
Ficure 4. Exterior of ventral valve, the type specimen. U.S. Nat. Mus. Cat. No. 52279a. 
4a.'Partly exfoliated ventral valve. U.S. Nat. Mus. Cat. No. 52279b. 
4b. Cast of interior of ventral valve and side outline. U.S. Nat. Mus. Cat. No. 52279e. 
4c. Cast of imperfect dorsal valve. U.S. Nat. Mus. Cat. No. 52279d. 
The specimens represented are all from Locality 1492, Middle Cambrian limestone at the forks of Pole Creek, 
above Cherry Creek Basin, Threeforks quadrangle (U. 8. Geol. Survey), Madison County, Montana. 
BILLINGSELLA WHITFIELDI (Walcott) (p. 764). 
Ficure 5. Exterior of ventral valve, with outline restored, from Locality 55, Middle Cambrian Eldorado limestone 
[Walcott, 1908f, p. 184], east slope of Prospect Mountain, in New York Canyon, Eureka district, 
Nevada. U.S. Nat. Mus. Cat. No. 24591a. 
5a. Exterior of dorsal valve from Locality 55b, Middle Cambrian Eldorado limestone, west side of Secret 
Canyon, Eureka district, Nevada. U.S. Nat. Mus. Cat. No. 57041la. This is the type specimen. 
The figures (5 and 5a) are redrawn from the specimens illustrated by Walcott [1884b, Pl. IX, figs. 4, 4a—b]. 
BILLINGSELLA oBscuRA Walcott (p. 758). 
Ficure 6. Exterior of an imperfect dorsal valve from Locality 74, Middle Cambrian sandstone about 300 feet (91.4 m. 
above the base of the Tonto group at the head of Nunkoweap Valley, Grand Canyon of the Colo- 
rado, Arizona. U.S. Nat. Mus. Cat. No. 52258a. This is the type specimen. 
62667°—vou 52, pr 2—12—_19 
