PLATE LXVII. 227 
ACROTRETA ARGENTA Walcott (p. 674). 
Fiaurgs 5, 5’, and 5’. Top, back, and side views of a compressed ventral valve in which the foraminal tube is exposed 
by the removal of the apex. U.S. Nat. Mus. Cat. No. 52062a. 
5a. Interior of a ventral valve. U.S. Nat. Mus. Cat. No. 52062b. 
5b and 5b’. Cast of a ventral valve that preserves the outline of the visceral cavity and the trapezoidal 
- area that includes the central, outside lateral, and middle lateral muscle scars. It is the type speci- 
men. U.S. Nat. Mus. Cat. No. 52062c. 
5c. Partly exfoliated dorsal valve. U.S. Nat. Mus, Cat. No. 52062d. 
5d. Top view of a crushed ventral valve. The dark portion preserves the shell. U.S. Nat. Mus. Cat. 
No. 52062e. 
The specimens represented by figures 5, 5a-d are from Locality 8b’, Upper Cambrian arenaceous shales, 1 mile 
southwest of Emigrant Pass, Esmeralda County, Nevada. 
Fiaures 5e and 5e’. Top and side views of a ventral valve from Locality 7y, Upper Cambrian limestone, 2.5 miles 
southeast of Emigrant Pass, Esmeralda County, Nevada. U.S. Nat. Mus. Cat. No. 35265a. 
5f and 5g. Interior and cast of dorsal valves from Locality 7x, Upper Cambrian limestone, 2.5 miles south- 
east of Emigrant Pass, Esmeralda County, Nevada. U.S. Nat. Mus. Cat. Nos. 35265b and 35265c, 
respectively. 
