PLATE LXV. 
a. Area. h. Central scars. 
a’. Outer lateral space of area. s. Median septum or ridge. 
el. Cardinal scars. vs. Main vascular sinus. 
fa. False area. x’. Cast of apical callosity. 
fp. False pedicle groove. 
ACROTRETA IDAHOENSIS Walcott (p. 687). 
[Pl. LXVIII, figs. 2, 2a-g.] 
Ficures 1, l’, 1’; la, la’, la’’; 1b, 1b’, 1b”; and 1c, le’, le’. Top. side, and back views of typical forms of the ventral 
valve. U.S. Nat. Mus. Cat. Nos. 52091a, 52091b, 52091c, and 52091d, respectively. Figure 1b repre- 
sents the type specimen. 
1d, 1d’, and 1d”. Top, back, and side views of cast of the interior of a ventral valve. U.S. Nat. Mus. Cat. 
No. 52091e. 
le. Cast of the interior of a dorsal valve. U.S. Nat. Mus. Cat. No. 52091f. 
li. Interior of a dorsal valve. U.S. Nat. Mus. Cat. No. 52091g. ; 
lg. Natural cast of the interior of the dorsal valve represented by figure 1f. U.S. Nat. Mus. Cat. No. 52091g. 
Jhand li. Exteriors of two dorsal valves. U.S. Nat. Mus. Cat. Nos. 52091h and 52091i, respectively. 
The specimens represented are all from Locality 5¢, Upper Cambrian limestone in Twomile Canyon, 2 miles south- 
east of Malade, Idaho. 
ACROTRETA ATTENUATA var. Walcott (p. 676). 
Ficures 2, 2’, and 2’’. Top, side, and back views of a specimen with a more distinctly marked false area than usually 
occurs in typical A. attenuata. Associated with the type of that species in Locality 302, Middle Cam- 
brian limestone east of West Gallatin (Gallatin) River above Gallatin, Montana. U.S. Nat. Mus. 
Cat. No. 52071a. 
ACROTRETA KUTORGAI Walcott (p. 692). 
[Text fig. 58, p. 692.] 
Ficures 3, 3’, 3, and 3a, 3a’, 3a’’. Top, back, and side views of the exterior of two ventral valves from Locality 139a, 
Middle Cambrian limestone near Chepultepec, Jefferson County, Alabama. U.S. Nat. Mus. Cat. 
Nos. 58305a and 58305b. 
3b, 3b’, 3b”, and 8c, 3c’, 3c’. Top, back, and side views of casts of the interior of ventral valves. U.S. Nat. 
Mus. Cat. Nos. 35277a and 35277b, respectively. Figure 3b represents the type specimen. 
3d. Side view of cast of ventral valve, showing cast of pedicle tube. U.S. Nat. Mus. Cat. No. 35277c. 
3e. Cast of pedicle tube, showing lines of shell growth. U.S. Nat. Mus. Cat. No. 35277d. 
3g. View into cast of ventral valve, with cast of area of dorsal valve in position. U.S. Nat. Mus. Cat. No. 
35277e. This specimen is tipped so as to show the cast of the areas of both the ventral and dorsal 
valves in text figure 58, page 692. 
3h, 3i, 3j, and 3k. Casts of interiors of dorsal valves, illustrating variation in median ridge, muscle scars, etc. 
U.S. Nat. Mus. Cat. Nos. 352771, 35277g, 35277h, and 352771, respectively. 
The specimens represented by figures 3b-k are all from Locality 90x, Middle Cambrian siliceous nodules in the 
Conasauga shale, Coosa Valley, Cherokee County, Alabama. 
AcROTRETA RUDIS Walcott (p. 702). 
[Pl. LXV, fig. 1.] 
Ficure 3f. Ventral valve, the type specimen, flattened in the shale of Locality 101a, Middle Cambrian, 3.5 miles south- 
west of Rogersyille, Hawkins County, Tennessee. U.S. Nat. Mus. Cat. No. 52111a. Copied from 
Walcott [1908d, Pl. IX, fig. 5]. 
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