PLATE XI. 
a. Area. h. Central muscle scar. 
ce. Trapezoidal area, including central, middle j- Anterior lateral muscle scar. 
lateral, and outside lateral muscle scars. 
OBOLUS TETONENSIS NINUS Walcott (p. 418). 
(Pl. VIII, figs. 11-0.] 
Ficures 1 and 1’. Top and side views of exterior of ventral valve, the type specimen. U. S. Nat. Mus. Cat. 
No. 51648a. 
la. Partly exfoliated ventral valve. U.S. Nat. Mus. Cat. No. 51643b. 
1b. Interior of ventral valve, showing outline of visceral area. U.S. Nat. Mus. Cat. No. 51643c. 
le. Dorsal valve, showing lamellation of shell. U.S. Nat. Mus. Cat. No. 51643d. 
ld and 1d’. Exfoliated dorsal valve, showing striation of interior layers of shell. U.S. Nat. Mus. Cat. No. 
51643e. 
Je and 1f. Exfoliated dorsal valves, showing character of interior layers of shell and variation in form from 
specimens represented by figures lcand1d. U.S. Nat. Mus. Cat. Nos. 51643f and 516438¢, respectively. 
lg. Cast of broken dorsal valve, showing anterior position of visceral area, and the central and anterior lateral 
sears. U.S. Nat. Mus. Cat. No. 51643h. 
The specimens represented are all from Locality 9r, Upper Cambrian limestone in the Reagan sandstone, Coman- 
che County, Oklahoma. 
Oxsotus (WxsToNtIA) THEMIS Walcott (p. 466). 
Ficure 2. Exterior of ventral valve, the type specimen, showing broken edges of lamelle of the shell. U.S. Nat. 
Mus. Cat. No. 51732a. 
2a. Dorsal valve, showing the same shell characters as figure 2. U.S. Nat. Mus. Cat. No. 51732b. 
2b. Broken ventral valve, showing cast of the visceral cavity and main vascular trunks. U.S. Nat. Mus. 
Cat. No. 51732c. 
2c. Partly exfoliated dorsal valve, posterior margin broken away. U.S. Nat. Mus. Cat. No. 51732d. 
The specimens represented are all from Locality 17e, Middle Cambrian sandy limestone on Grand View trail, 
south side Grand Canyon of the Colorado, Arizona. 
OBOLUS ISMENE Walcott (p. 393). 
Ficure 3. Portion of ventral valve, the type specimen, showing cardinal slopes and the striation of one of the inner 
layers of shell. U.S. Nat. Mus. Cat. No. 51545a. 
3a. Partly exfoliated dorsal valve. U.S. Nat. Mus. Cat. No. 51545b. 
3b. Interior of a dorsal valve showing area. U.S. Nat. Mus. Cat. No. 51545c. 
The specimens represented are all from Locality 369, Upper Cambrian sandstone in the Elvins formation, Flat 
River, St. Francois County, Missouri. 
OxsoLtus NuNDINA Walcott (p. 406). 
Figure 4. Ventral valve, the type specimen, showing the characteristic strong concentric ridges. 
4a. Partly exfoliated dorsal valve. 
The specimens represented are on the same piece of limestone from Locality 14b, Upper Cambrian, Cold Creek, 
Llano County, Texas. U.S. Nat. Mus. Cat. Nos. 51587a and 51587b, respectively. 
OBoLus acapicus Walcott (p. 380). 
Figure 5. Cast of the interior of a dorsal valve, the type specimen, from Locality 3h, Upper Cambrian shale, McNeil 
Brook, Cape Breton, Nova Scotia. U.S. Nat. Mus. Cat. No. 51540a. The posterior portion of the 
shell was restored from another specimen (Catalogue No. 51540b), the two being counterparts. 
OBOLUS PHERES Walcott (p. 408). 
Figures 6 and 6a. Views of partly exfoliated ventral valves, showing thickness of the lamellated shell. U.S. Nat. 
Mus. Cat. No. 51589a and 51589b, respectively. Figure 6 represents the type specimen. 
6b. Imperfect dorsal valve. U.S. Nat. Mus. Cat. No. 51589c. 
The specimens represented are all from Locality 10v, Upper Cambrian shale near Baraboo, Wisconsin. 
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