PLATE LXIX. 
cl. Cardinal scar. s. Median septum or ridge. 
h. Central sear. vs. Main vascular sinus. 
ACROTRETA PRIMHVA Walcott (p. 700). 
Froures 1, 1’, 1”; Ja, la’, la’; and 1b, 1b’, 1b”. Top, side, and back views of three ventral valves. U.S. Nat. 
Mus. Cat. Nos. 15343a, 15342a, and 15342b, respectively. The type specimen, represented by 
figure 1, was figured by Walcott [1886b, Pl. VIII, figs. 1 and la] as Acrotreta gemma. These figures 
have been repeatedly copied (see p. 700). 
Je. Exterior of dorsal valve. U.S. Nat. Mus. Cat. No. 15343b. Specimen figured by Walcott [1886b, Pl. VIII, 
fig. lb] as Acrotreta gemma. ‘This figure has been repeatedly copied (see p. 700). 
1d. Cast of the interior of a dorsal valve. U.S. Nat. Mus. Cat. No. 15342c. 
le, le’, and le’. Top, side, and back views of a cast of the interior of a ventral valve. U.S. Nat. Mus. 
Cat. No. 15342d. 
lf. Cast of the interior of a dorsal valve. U.S. Nat. Mus. Cat. No. 5206la. 
The specimens represented are all from Locality 31a, Lower Cambrian limestone near Pioche, Nevada. 
AOROTRETA PYXIDICULA White (p. 701). 
Ficure 2. Dorsal valve irom Locality 313f, Upper Cambrian limestone at Schellbourne, Nevada. U. 8. Nat. Mus. 
Cat. No. 8570a. Specimen figured by White [1877, Pl. III, figs. 3a—b(?)]. 
2a and 2b. Exterior and outline of convexity of two dorsal valves from Locality 4h, Middle Cambrian lime- 
stone 4 miles east-northeast of Logan, Gallatin County, Montana. U.S. Nat. Mus. Cat. Nos. 52149a 
and 52149b, respectively. 
Qc, 2c’, 2c’; 2d, 2d’, 2d’; and 2e, 2e’, 2e’’. Top, side, and back views of three ventral valves associated 
with the dorsal valves represented by figures 2a and 2b. U.S. Nat. Mus. Cat. Nos. 52149c, 52149d, 
and 52149e, respectively. 
2%, 27, and 2”. Top, side, and back views of a ventral valve associated with the specimen represented by 
figure 2. U.S. Nat. Mus. Cat. No. 8570b. Type specimen, figured by White [1877, Pl. III, figs. 3c 
and 3d (?)]. 
8, 3’, and 3’. Top, side, and back views of ventral valve from Locality 5b, Middle Cambrian Langston lime- 
stone, 2 miles southeast of Malade, Oneida County, Idaho. U.S. Nat. Mus. Cat. No. 52147a. 
3a, 3a’, and 3a’’. Top, side, and back views of cast of the interior of a ventral valve associated with the speci- 
men represented by figure 3. U.S. Nat. Mus. Cat. No. 52147b. 
3b. Top and side views of cast of the interior of a dorsal valve associated with the specimen represented by 
figure 3, in which the median ridge is slightly developed. U.S. Nat. Mus. Cat. No. 52147c. 
3c and 3c’. Top, side, and back views of cast of the interior of a dorsal valve associated with the specimen 
represented by figure 3, showing strong median ridge and central and cardinal scars. U.S. Nat. 
Mus. Cat. No. 52147d. 
3d. Exterior and side outline of convexity of a dorsal valve from Locality 322, Middle Cambrian limestone 15 
miles west of Montpelier, Idaho. U.S. Nat. Mus. Cat. No. 52150a. 
3e, 3e’, and 3e”. Top, side, and back Views of a cast of the interior of a ventral valve associated with the 
specimen represented by figure 3. U.S. Nat. Mus. Cat. No. 52147e. 
3f, 31, and 31’. Top, side, and back views of the interior of a ventral valve associated with the specimen 
represented by figure 3d. U.S. Nat. Mus. Cat. No. 52150b. 
ACROTRETA SIGNALIS Walcott (p. 711). 
Ficures 4, 4’, and 4”. Top, side, and back views of a ventral valve, the type specimen, from Locality 328e, Middle 
Cambrian sandstone at St. Croix Falls, Polk County, Wisconsin. The foraminal aperture of 4/ is 
restored from a second specimen. U.S. Nat. Mus. Cat. No. 35285a. 
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