PLATE LXVILI. 
el. Cardinal scar. s. Median septum. 
fa. False area. K v. Visceral cavity. 
{p. False pedicle groove. x. Foraminal aperture. 
h. Central muscle scar. x’. Cast of foraminal aperture. 
1. Outside lateral muscle scar. 
ACROTRETA miIcROscoPica (Shumard) (p. 693). 
Figures 1, V, 1’; la, la’, la’; 1b, 1b’, 1b’; and 1c, 1c’, 1c’. Top, back, and side views of the exterior of four ventral 
: valves illustrating variations in outline, position of the apex, etc. Locality 67, Upper Cambrian 
limestone, on Tatur Hill, northeast of Burnet, Burnet County, Texas. U.S. Nat. Mus. Cat. Nos. 
52120a, 52120f, 52120b, and 52120c, respectively. 
1d, 1d’, and 1d”. Top, back, and side views of a ventral valve from Locality 70e, Upper Cambrian limestone, 
near Morgans Creek, Burnet County, Texas. U.S. Nat. Mus. Cat. No. 52118a. 
le and lg. Exteriors of dorsal valves associated with the shells represented by figures 1, la~c. U.S. Nat. Mus. 
Cat. Nos. 52120d and 52120e, respectively. z 
lfand lh. Exteriors of dorsal valves associated with the specimen represented by figure 1d. U.S. Nat. Mus. 
Cat. No. 52118b and 52118c, respectively. 
lo, lo’, and lo’. Top, side, and back views of the cast of a small ventral valve from Locality 68, Upper Cam- 
brian limestone, on Packsaddle Mountain, Llano County, Texas. U.S. Nat. Mus. Cat. No. 52504a. 
2, 2’, and 2’. Top, back, and side views of a ventral valve with an unusually well-defined false area. U.S. 
Nat. Mus. Cat. No. 52119a. 
2a, 2a’, and 2a/’. Top, back, and side views of a ventral valve which has lost the apex and exposed the forami- 
naltube. U.S. Nat. Mus. Cat. No. 52119b. 
2b and 2c. Side views of two elevated ventral valves. U.S. Nat. Mus. Cat. Nos. 52119c and 52119d, respec- 
tively. 
2d. Exterior of dorsal valve with side outline. U.S. Nat. Mus. Cat. No. 52119e. 
The specimens represented by figures 2, 2a-d are all from Locality 80, Upper Cambrian limestone, 2 miles north 
of Aurum, Schell Creek Range, White Pine County, Nevada. 
ACROTRETA MICROSCOPICA TETONENSIS Walcott (p. 694). 
Figures 3, 3’, 3/’; 3a, 3a’, 3a”; and 3b, 3b’, 3b”. Top, back, and side views of three ventral valves. U.S. Nat. Mus. 
Cat. Nos. 35280a, 35280b, and 35280e, respectively. Figure 3 represents the type specimen. 
3c and 3d. Exterior views, and outlines showing convexity, of two dorsal valves. U.S. Nat. Mus. Cat. Nos. 
35280c and 35280d, respectively. 
The specimens represented by figures 3, 3a-d are from Locality 4e, Middle Cambrian limestones in divide at the 
head of Jackson Creek, Teton Mountains, Wyoming. 
ACROTRETA MICROSCOPICA MISSOURIENSIS Walcott (p. 694). 
Fiaures 4, 4’, and 4”. Top, back, and side views of the type specimen, a ventral valve that has been slightly com- 
pressed longitudinally, thus elevating it, also emphasizing the median furrow on the false area. 
U.S. Nat. Mus. Cat. No. 35728a. : 
4a and 4a’. Top and side views of a ventral valve that: has been depressed by compression, the apex being 
forced backward over the false area. U.S. Nat. Mus. Cat. No. 35278b. 
4b. Exterior of dorsal valve, showing traces of radiating strize and numerous indentations caused by the fine 
grains of sand in the matrix. U.S. Nat. Mus. Cat. No. 35278c. 
The specimens represented are all from Locality 11k, Middle Cambrian shale, St. Francois County, Missouri. 
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