PLATE LXIV. 
a. Outer lateral space of area. h. Central scars. 
cl. Cardinal scars. s. Median septum or ridge. 
{. Flexure line of area. y. Visceral area. 
fa. False area. vs. Main vascular sinus. 
p. False pedicle groove. x’’. Cast of apical callosity. 
ACROTRETA ATTENUATA Meek (p. 675). 
[Pl. LXXIX, figs. 6 and 6a.] 
Fieures 1, 1’, 1’, and la, la’, la’’. Top views with side and back views of ventral valves in the material upon which 
Meek founded the species. From Locality 302, Middle Cambrian limestone east of West Gallatin 
(Gallatin) River, above Gallatin, Montana. U.S. Nat. Mus. Cat. Nos. 7867a and 7867b, respectively. 
Figure 1 represents the type specimen. ; ; 
lb, 1b’, 1b’, and Ic, le’, 1c’. Top, side, and back views of two ventral valves from Locality 156a, Middle 
Cambrian limestones north of East Gallatin River, near Hillsdale, Montana. U.S. Nat. Mus. Cat. 
Nos. 52063a and 52063b, respectively. 
1d, 1d’, 1d’’; le, le’, le’; and 1f, 1f’, 1f/”. Top, side, and back views of three ventral valves, from Locality 4n, 
Middle Cambrian limestone in the divide at the head of Jackson Creek, Teton Mountains, Wyoming. 
U.S. Nat. Mus. Cat. Nos. 52067a, 52067b, and 52067c, respectively. 
lg, lg’, and 1g”. Top, side, and back views of a ventral valve from Locality 159, Middle Cambrian limestone 
on the north side of West Gallatin (Gallatin) River, Montana. U.S. Nat. Mus. Cat. No. 52064a. 
Ih. Dorsal valve associated with the ventral valves represented by figures 1 and la. U.S. Nat. Mus. Cat. 
No. 7867c. 
li. Interior of ventral valve, showing base of apical callosity, vascular sinuses, and cardinal scars. From 
p Locality 10y, Middle Cambrian Marjum limestone, southeast of Marjum Pass, House Range, Utah. 
U.S. Nat. Mus. Cat. No. 52070a. 
1j, 1k, and 11. Top, back, and side views of a cast of the interior of a ventral valve associated with the shell 
represented by figure li. U.S. Nat. Mus. Cat. No. 52070b. 
1m. Interior of a dorsal valve associated with the specimen represented by figure li. U.S. Nat. Mus. Cat. 
No. 52070ce. 
In. Interior of a dorsal valve from Locality 3s, Middle Cambrian shales of the Wheeler formation, east of Ante- 
lope Springs, House Range, Utah. U.S. Nat. Mus. Cat. No. 52069a. 
lo. Exterior of a dorsal valve associated with the specimen represented by figure In. U.S. Nat. Mus. Cat. 
No. 52069b. : 
ACROTRETA DEFINITA Walcott (p. 683). 
FicurEs 2, 2’, 2’; 2a, 2a’, 2a/’; and 2b, 2b’, 2b’. Top, side, and back views of typical forms of the ventral valve. 
U.S. Nat. Mus. Cat. Nos. 35270a, 35270b, and 35270c, respectively. 
2c, 2c’, and 2c’”. Cast of the interior of a ventral valve, the type specimen, with back and side views. 
U.S. Nat. Mus. Cat. No. 35270d. 
2d, 2d’, and 2d’’. Top, back, and side views of the cast of the interior of a dorsal valve, showing the area, 
cardinal scar, etc., with unusual clearness. U.S. Nat. Mus. Cat. No. 35270e. 
2e, 2i, and 2g. Exterior views of three dorsal valves, illustrating variation in outline. U.S. Nat. Mus. Cat. 
Nos. 35270, 35270g, and 35270h, respectively. . 
The specimens represented by figures 2, 2a-g are from Locality 163, Middle Cambrian Spence shale member of 
Ute limestone, Spence Gulch, 15 miles west of Montpelier, Idaho. 
FiaurEs 3, 3a, and 3b. Top, back, and side views of a cast of a ventral valve from Locality 55, Middle Cambrian 
Eldorado limestone in New York Canyon, Eureka district, Nevada. U. S. Nat. Mus. Cat. No. 
35271a. 
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