PLATE LXXIV. 
a. Area. vy. Visceral area. 
cl. Cardinal sear. vs. Main vascular sinus, 
h. Central sear. x’’, Cast of apical callosity. 
s. Median septum. 
ACROTRETA OPHIRENSIS Walcott (p. 697). 
Ficurss 1, 1’, and 1”. Top, side, and back views of ventral valve, the type specimen. U.S. Nat. Mus. Cat. No. 
35281q. 
la, la’, la’; 1b, 1b’, 1b’’; and lc, 1c’, 1c’. Top, side, and back views of ventral valves that illustrate 
variation in form. U.S. Nat. Mus. Cat. Nos. 3528la-c, respectively. 
1d, 1d’, and 1d’. Top, back, and side views of cast of interior of a ventral valve. U.S. Nat. Mus. Cat. No. 
35281d. 
le. Cast of the interior of a ventral valve, showing outlines of visceral cavity. U.S. Nat. Mus. Cat. No: 
35281e. 
1f. Interior of ventral valve with area preserved. U.S. Nat. Mus. Cat. No. 35281f. 
lg. Cast of interior of a dorsal valve. U.S. Nat. Mus. Cat. No. 35281g. 
lh. Cast of part of the interior of a dorsal valve broadened by distortion. U.S. Nat. Mus. Cat. No. 35281h. 
li. Cast of the interior of a dorsal valve narrowed by distortion. U.S. Nat. Mus. Cat. No. 352811. 
1j, 1j’, and 1j’. Top, side, and back views of exterior of dorsal valve. U.S. Nat: Mus. Cat. No. 35281). 
1k and 1k’. Top and side views of a dorsal valve. U.S. Nat. Mus. Cat. No. 35281k. 
11. Interior of a dorsal valve. U.S. Nat. Mus. Cat. No. 352811. 
1m, lm’, 1m”, and In, In’, 1n”. Top, side, and back views of casts of interior of elongated dorsal valves. 
U.S. Nat. Mus. Cat. Nos. 35281m and 35281n, respectively. 
lo. Interior of a finely preserved dorsal valve. U.S. Nat. Mus. Cat. No. 352810. 
lp. Cast of the interior of an undistorted dorsal valve. U.S. Nat. Mus. Cat. No. 35281p. 
The specimens represented are all from Locality 3e, Middle Cambrian limestone less than 400 feet (121.9 m.) above 
the quartzitic sandstones, at Ophir, Oquirrh Range, Tooele County, Utah. 
ACROTRETA OPHIRENSIS RUGOSA Walcott (p. 699). 
Figure 2. Interior of a ventral valve. U.S. Nat. Mus. Cat. No. 35282a. 
2a and 2a’. Top and side views of a ventral valve, the type specimen. U.S. Nat. Mus. Cat. No. 35282b. 
2b and 2b’. Top and side views of a cast of the interior of a ventral valve. U.S. Nat. Mus. Cat. No. 35282c. 
2c and 2c’. Top and side views of a dorsal valve. U.S. Nat. Mus. Cat. No. 35282d. 
2d. Interior of a dorsal valve. U.S. Nat. Mus. Cat. No. 35282e. 
2e. Cast of interior of a dorsal valve. U.S. Nat. Mus. Cat. No. 35282f. 
The specimens represented are all from Locality 3e, Middle Cambrian limestone less than 400 feet (121.9 m.) above 
the quartzitic sandstones, at Ophir, Oquirrh Range, Tooele County, Utah. 
ACROTRETA CLAYTONI Walcott (p. 680). 
Fiaures 3, 3’, and 3’. Top, side, and back views of a ventral valve from Locality 175, Lower Cambrian shales on the 
divide between Clayton and Fish Lake valleys, about 15 miles (24.2 km.) south-southwest of the town 
of Silver Peak, Silver Peak quadrangle (U.S. Geol. Survey), Esmeralda County, Nevada. U.S. Nat. 
Mus. Cat. No. 52080a. This is the type specimen. 
3a. Partly exfoliated dorsal valve associated with the specimen represented by figure 3. U.S. Nat. Mus. 
Cat. No. 52080b. 
3b. Cast of interior of a distorted ventral valve from Locality 11, Lower Cambrian limestones of No. 3 of the 
Silver Peak group [Walcott, 1908f, p. 189], about 2.5 miles (4 km.) south of Barrel Spring, and 0.5 
mile (0.8 km.) east of the road, in the extreme southeastern corner of the Silver Peak quadrangle 
(U.S. Geol. Survey), Esmeralda County, Nevada. U.S. Nat. Mus. Cat. No. 5208la. 
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