PLATE LXVIII. 
cl. Cardinal scar. s. Median septum or ridge. 
fa. False area. v. Visceral area. 
fp. False pedicle groove. ys. Main vascular sinus. 
h. Central muscle scar. 
Acrorreta curvaTa Walcott (p. 682). 
Ficures 1, 1’, 1”; 1a, la’, 1a’’; and 1b, 1b’, 1b’. Top, side, and back views of the exterior of ventral valves. U.S. 
Nat. Mus. Cat. Nos. 35269a, 35269b, and 35269c, respectively. Figure larepresents the type specimen. 
le; le’, le’; 1d, 1d’, 1d’; and le, le’, le’. Top, side, and back views of casts of the interior of ventral 
valves. U.S. Nat. Mus. Cat. Nos. 35269d, 35269e, and 35269f, respectively. 
1f, 1g, 1h, and 1i. Exterior and outline of convexity of four dorsal valves. U.S. Nat. Mus. Cat. Nos. 35269g, 
35269h, 35269i, and 35269j, respectively. The specimen represented by figure 1g was figured by Walcott 
[1884b, Pl. I, fig. 1d]as Acrotreta gemma. The specimen has been repeatedly figured (see p. 682). 
1k and 11. Interiors of two dorsal valves. U.S. Nat. Mus. Cat. Nos. 35269k and 352691, respectively. The 
specimen represented by figure 11 was figured by Walcott [1884b, Pl. I, fig. le] as Acrotreta gemma. 
The specimen has been repeatedly figured (see p. 682). 
1m, 1m’, 1m”, and 1n, In’, In”. Top, side, and back views of casts of the interior of dorsal valves. U.S. 
Nat. Mus. Cat. Nos. 35269m and 35269n, respectively. 
The specimens represented are all from Locality 208a, passage beds between the Upper Cambrian and the Ordovi- 
cian, Hamburg Ridge, southwest of Wood Cone, Eureka district, Nevada. 
ACROTRETA IDAHOENSIS Walcott (p. 687). 
[Pl. LXV, figs. 1, 1a-i.] 
RIGURES!2, 242/529) 2a, 2a/*: Db, 2b% 27: and 2c, 2c’, 2c’”. Top, side, and back views of four ventral valves. U.S. 
Nat. Mus. Cat. Nos. 52089a, 52089b, 52089c, and 52089d, respectively. 
2d, 2e, and 2f. Exterior and outline of convexity of three dorsal valves. U.S. Nat. Mus. Cat. Nos. 52089e, 
52089f, and 52089g, respectively. 
The specimens represented by figures 2, 2a-f are from Locality 88a, Middle Cambrian limestone in the northern 
suburbs of Deadwood, Black Hills, South Dakota. 
Fieure 2¢. Ventral valve from Locality 165, Middle Cambrian limestone, about 1 mile below the main part of Dead- 
° wood, Black Hills, South Dakota. U.S. Nat. Mus. Cat. No. 20908. 
ACROTRETA LISANI Walcott (p. 692). 
Fieures 3, 3a, and 3b. Top, back, and side views of a ventral valve, the type specimen. U.S. Nat. Mus. Cat. 
No. 52505a. 
3c. Exterior of dorsal valve. U.S. Nat. Mus. Cat. No. 52505b. 
The specimens represented by figures 3, 3a-c are from Locality €22, Middle Cambrian, Changhia limestone, at 
Changhia, Shantung, China. 
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