PLATE XXIII. 
a. Central lateral space of area. j. Anterior lateral muscle sear. 
a’. Outer lateral space of area. p. Pedicle groove. . 
f, Plexure lines of area. s. Median septum. 
h. Central muscle sear. vy. Visceral cavity. 
i. Transmedian muscle sear. vs. Main vascular sinus. 
Osotus wituist (Walcott) (p. 420). 
FicurE 1. Cast of a ventral valve, the type specimen, compressed in shale, Locality 140a, Middle Cambrian, near 
Cave Spring, Floyd County, Georgia. U.S. Nat. Mus. Cat. No. 27345a. 
la. Cast of a ventral valve compressed in shale, Locality 107b, Middle Cambrian, near Copper Ridge, 11 miles 
northwest of Knoxville, Tennessee. U.S. Nat. Mus. Cat. No. 27343a. 
1b. Cast of a ventral valve compressed in shale, that shows the pitted or punctate character of the interior of the 
shell. Same locality as specimen represented by figure la. U.S. Nat. Mus. Cat. No. 273438b. 
le. Cast of the outer surface of a ventral valve, from same band of shale as specimen represented by figure la. 
U.S. Nat. Mus. Cat. No. 27348c. 
1d. Cast of the outer surface of a dorsal valve associated with specimen represented by figure la. U.S. Nat. 
Mus. Cat. No. 27343d. 
le. Cast of a ventral valve distorted by compression in the same band of shale as the specimen represented by 
figure la. U.S. Nat. Mus. Cat. No. 27343e. 
li. Cast showing part of a ventral valve, the outline of which ‘is restored from associated specimens. It occurs 
in the calcareous sandstone embedded in the shales of Locality 107b, Middle Cambrian, near Copper 
Ridge, 11 miles northwest of Knoxville, Tennessee. U.S. Nat. Mus. Cat. No. 27343f. 
lg. Cast of the interior of a dorsal valve associated with the specimen represented by figure lf. U.S. Nat. 
Mus. Cat. No. 27348¢. 
th. Cast of the interior of a dorsal valve associated with specimen represented by figure 1f but which is more 
transverse. U.S. Nat. Mus. Cat. No. 27343h. 
li. Distorted cast of a ventral valve from Locality 101b, Middle Cambrian shale 3.5 miles southwest of Rogers- 
ville, Tennessee. U.S. Nat. Mus. Cat, No. 27344a. 
1j. Flattened ventral valve, showing the strong interrupted radiating lines and the strong concentric rugose 
lines of growth. Locality 93, Upper Cambrian shale, near Jordans on Cowan Creek, Cherokee County, 
‘Alabama. U.S. Nat. Mus. Cat. No. 27346a. 
LINGULELLA TARPA (Walcott) (p. 534). 
Fiaure 2. Exfoliated ventral valve from sandy shales, which shows general outline and some of the markings of the 
interior of the shell. U.S. Nat. Mus. Cat. No. 27342a. 
2a. Compressed, slightly distorted ventral valve, the type specimen, from argillaceous shales interbedded 
with the sandy shales in which¢the specimen represented by figure 2 occurs. U.S. Nat. Mus. Cat. 
No. 27342b. 
2b. Cast of a dorsal valve, from calcareous layers in the sandy shale in which specimen represented by figure 2 
occurs. U.S. Nat. Mus. Cat. No. 27342c. 
2c. Somewhat more elongate shell than that represented by figure 2b, with which it is associated, and which 
is somewhat doubtfully identified as a dorsal valve. U.S. Nat. Mus. Cat. No. 27342d. 
The specimens represented are all from Locality 11, Middle Cambrian, 1 mile east of Post Oak Springs, Roane 
County, Tennessee. 
OBOLUS MCCONNELLI (Walcott) (p. 396). 
Ficure 3. Interior of a crushed and broken ventral valve, the type specimen, from Locality 14s, Middle Cambrian 
Ogygopsis zone on Mount Stephen, British Columbia. U.S. Nat. Mus. Cat. No. 51572a. 
3a. Interior of an associated fragmentary ventral valve of the broad form. U.S. Nat. Mus. Cat. No. 51572b. 
Copied from Walcott [1908c, Pl. I, fig. 2]. (See note under fig. 3d, below.) 
OBOLUS MCCONNELLI PELIAS (Walcott) (p. 398). 
(Pl. XXXIX, figs. 5, 5a-c.] 
Figure 3b. Interior of a broad form of dorsal valve, the type specimen, from Locality 11q, Middle Cambrian Marjum 
limestone, ridge east of Wheeler Amphitheater, House Range, Utah. U.S. Nat. Mus. Cat. No. 51592a. 
3c. Interior of an associated dorsal valve. U.S. Nat. Mus. Cat. No. 51592b. 
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