PLATE XXV. 
a. Central lateral space of area. p. Pedicle groove. 
a’. Outer lateral space of area. ps. Parietal band. 
f. Flexure line of area. s. Median septum. 
h. Central muscle scar. v. Visceral cavity. 
i. Transmedian muscle scar. vs. Main vascular sinus. 
j. Anterior lateral muscle sear. 
LINGULELLA HAYESI (Walcott) (p. 505). 
Figure 1. Cast of the interior of a ventral valve, showing the area and portions of the visceral cavity. U.S. Nat. 
Mus. Cat. No. 27319}. 
la. Interior of a partly weathered ventral valve. U.S. Nat. Mus. Cat. No. 27319a. 
1b and 1c. Casts of the interior of the ventral valve, showing the position of the main vascular sinuses, the 
visceral cavity, and the area. U.S. Nat. Mus. Cat. Nos. 27319b and 27319c, respectively. Figure 
1b represents the type specimen. 
1d. Enlargement of the cast of the area of the ventral valve, showing the pedicle groove, the flexure line, and 
the central and outer lateral spaces of the area. U.S. Nat. Mus. Cat. No. 27319d. 
le. Cast of the interior of a dorsal valve showing the position of the central (h) and anterior lateral (j) muscle 
scars, the scar of the parietal band (ps), also the arching forward of the center of the area. U.S. Nat. 
Mus. Cat. No. 27319e. 
lf, 1g, 1h, and li. Casts of the interior of dorsal valves, showing variation in form of the valve and in the posi- 
tion of the parietal band and main vascular sinus. The latter is especially well shown by figure 1g. 
U.S. Nat. Mus. Cat. Nos. 27319f, 27319¢, 27319h, and 27319i, respectively. 
1j. Enlargement of the area of the dorsal valve represented by figure 1f. U.S. Nat. Mus. Cat. No. 27319f. 
All of the specimens represented occur on the exterior of siliceous nodules, or on breaks across them, from Locality 
90x, Middle Cambrian Conasauga shale, Coosa Valley, Cherokee County, Alabama. 
Opotus (WESTONIA) CHUARENSIS (Walcott) (p. 454). 
Figures 2 and 2a. Partly exfoliated ventral valves from Locality 78a, Middle Cambrian sandstone, Chuar Valley, 
Grand Canyon of the Colorado, Arizona, showing some variation in form. U.S. Nat. Mus. Cat. Nos. 
57020a and 57020b, respectively. Figure 2 represents the type specimen. 
2b. Crushed ventral valve from Locality 74b, Middle Cambrian sandstone in Nunkoweap Valley, Grand 
Canyon of the Colorado, Arizona. U.S. Nat. Mus. Cat. No. 57021a. 
2c. Exterior of a rather large dorsal valve from Locality 74, Middle Cambrian sandstone at the head of Nunko- 
weap Valley, Grand Canyon of the Colorado, Arizona. U.S. Nat. Mus. Cat. No. 27310a. 
2d. Cast of part of the interior of a dorsal valve associated with the shells represented by figures 2 and 2a, 
showing a median depression and the area. U.S. Nat. Mus. Cat. No. 57020c. 
2e. Strongly marked cast of the interior of a dorsal valve associated with the shell represented by figure 2c, 
showing the depression filled by the ridges adjacent to the main vascular sinuses, a strong central 
grooye corresponding to the median septum, and the central muscle scars (h). U. S. Nat. Mus. 
Cat. No. 27310b. 
Oxpoivs zETUS (Walcott) (p. 421). 
Figures 3, 3a, and 3b. Partly exfoliated ventral valves, showing the external form and something of the character 
of the different layers forming the shell. U.S. Nat. Mus. Cat. Nos. 27347a, 27347b, and 27347c, 
respectively. Figure 3a represents the type specimen. 
3c. Partly exfoliated dorsal valve. U.S. Nat. Mus. Cat. No. 27347d. 
The specimens represented are all from Locality 74, Middle Cambrian sandstone, near the head of Nunkoweap 
Valley, Grand Canyon of the Colorado, Arizona. 
OBOLUS LAMBORNI MINIMUS (Walcott) (p. 395). 
Ficures 4 and 4a. Casts of ventral valves that were compressed in the shale. U.S. Nat. Mus. Cat. Nos. 27322a and 
27322b, respectively. Figure 4 represents the type specimen. 
4b. Dorsal valve associated with the ventral valves represented by figures 4 and 4a. U.S. Nat. Mus. Cat. 
No. 27322c. 
The specimens illustrated are all from Locality 101a, Middle Cambrian Rogersville shale, 3.5 miles southwest of 
Rogersville, Hawkins County, Tennessee. 
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