PLATE XLVIL. 
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 sear. vs. Main vascular sinus. 
j. Anterior lateral muscle scar. 7 
Osotus (WesTonIA) ELLA (Hall and Whitfield) (p. 455). 
[Pl. XX XIII, figs. 3, 3a-c.] 
Figure 1. Ventral valve laterally compressed and flattened, from Locality 30a, Middle Cambrian siliceous shale in 
Big Cottonwood Canyon, southeast of Salt Lake City, Utah. U.S. Nat. Mus. Cat. No. 51691a. 
la. Dorsal valve associated in the same bed of shale with specimen represented by figure 1, but it is not com- 
pressed laterally. Neither specimen preserves any of the shell substance. U.S. Nat. Mus. Cat. No. 
51691b. ; 
lb. Enlarged figure of the type specimen, a dorsal valve, from Locality 329, Middle Cambrian shales near 
Calls Fort, Boxelder County, Utah. U.S. Nat. Mus. Cat. No. 1532la. Specimen figured by Hall 
and Whitfield [1877, Pl. I, fig. 8] as Lingulepis ella; and by Walcott [1886b, Pl. VIII, fig. 4; and 1891a, 
Pl. LXVII, fig. 2b] as Lingulella ella. 
lc. Cast of the interior of a slightly distorted ventral valve from Locality 329e, Middle Cambrian siliceous 
shales of East Canyon, above Ophir, Oquirrh Range, Tooele County, Utah. U.S. Nat. Mus. Cat. 
No. 15820a. 
1d. Another cast of the interior of a ventral valve that is associated with the specimen represented by figure 
1c, but which has suffered less from distortion. U.S. Nat. Mus. Cat. No. 15320b. Specimen figured 
by Walcott [1898b, Pl. XXVIII, fig. 7] as Obolus (Lingulelia) ella. 
le and 1f. Ventral and dorsal valves of young shells associated with the shells represented by figures 1 and 
la. U.S. Nat. Mus. Cat. Nos. 51691c and 51691d, respectively. 
lg. Impression of the interior of a ventral valve from Locality 302s, Middle Cambrian shale in Last Chance 
Gulch, south of Helena, Montana. U.S. Nat. Mus. Cat. No. 51686a. Specimen figured by Walcott 
[1898b, Pl. XXVIII, fig. 5] as Obolus (Lingulella) ella. 
1h and li. Interior casts of ventral and dorsal valves flattened in argillaceous shales of Locality 31, Middle 
Cambrian, at the Chisholm mine, near Pioche, Nevada. U.S. Nat. Mus. Cat. Nos. 15319a and 15319b, 
respectively. The specimen represented by figure 1i was figured by Walcott [1898b, Pl. XXVIII, 
fig. 8] as Obolus (Lingulella) ella. 
1j. Cast of the interior of a small ventral valve with a very high area, from Locality 4g, Middle Cambrian shale 
1 mile north of the forks of Eastand West Gallatin rivers, Montana. U.S. Nat. Mus. Cat. No. 51685a. 
1k. Impression of the interior of a slightly distorted dorsal valve from the same locality as specimen repre- 
sented by figure lg, showing the characteristic muscular impressions. U. 8. Nat. Mus. Cat. No. 
51686b. Specimen figured by Walcott [1898b, Pl. XXVIII, fig. 6] as Obolus (Lingulelia) ella. 
11. Interior of finely preserved dorsal valve from Locality 5h, Middle Cambrian shales 2 miles southeast of 
Malade, Idaho. U.S. Nat. Mus. Cat. No. 51689a. 
1m. Enlargement of a small individual associated with the shells represented by figures 1h and li, showing 
characteristic surface markings. U.S. Nat. Mus. Cat. No. 15319c. The counterpart of this specimen 
(Cat. No. 15319d) was used to aid in restoring certain portions of figure 1m. 
In. Cast of an interior of a ventral valve associated with the ventral valve represented by figure 1j. U.S. 
Nat. Mus. Cat. No. 51685b. 
lo. Enlargement by photography of the surface of a specimen associated with the shell represented by figure 
1j. U.S. Nat. Mus. Cat. No. 51685c. 
lp. Distorted ventral valve from Locality 90, Middle Cambrian shales near Craigs Mountain, Cherokee County, 
Alabama. U.S. Nat. Mus. Cat. No. 51634a. 
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