PLATE XXVIII. 
a. Central lateral space of area. p. Pedicle groove. 
a’. Outer latera! space of area. s. Median septum. 
f. Flexure line of area. v. Visceral cavity. 
h. Central muscle scar. vs. Main vascular sinus. 
i. Transmedian muscle scar. y- See text (p. 478). 
j. Anterior lateral muscle scar. 
LINGULELLA AMPLA (Owen) (p. 477). 
Fieure 1. Ventral valve partly broken away near the apex, from Locality 339e, Middle Cambrian sandstone on 
Mountain Island, Minnesota. U.S. Nat. Mus. Cat. No. 17859b. Type specimen, figured by Owen 
[1852, Pl. IB, fig. 5] as Lingula ampla. The specimen represented by figure 12 of the same plate 
(Catalogue No. 17859a) is not figured in this monograph. 
la. Ventral valve, from Locality 84s, Middle Cambrian sandstone near Dakota, Winona County, Minnesota. 
U.S. Nat. Mus. Cat. No. 35241a. 
1b. Dorsal valve, associated with specimen represented by figure la. U.S. Nat. Mus. Cat. No. 35241b. 
Ic. Cast of the interior of a ventral valve associated with the specimen represented by figure la. U.S. Nat. 
Mus. Cat. No. 35241c. ? 
1d. Cast of the interior of the ventral valve represented in figure 1f, enlarged. Locality 84s, Middle Cambrian 
sandstone near Dakota, Winona County, Minnesota. U.S. Nat. Mus. Cat. No. 35241d. 
le. Cast of the interior of the posterior portion of a ventral valve associated with the specimen represented by 
figure la. U.S. Nat. Mus. Cat. No. 35241e. 
1f. Cast of the interior of a ventral valve showing an unusual feature at y. This specimen is enlarged in 
figure ld. U.S. Nat. Mus. Cat. No. 35241d. 
1g. Cast of the interior of a dorsal valve associated with the specimen represented by figure la. U.S. Nat. 
Mus. Cat. No. 35241f. 
th. Cast of the interior of a dorsal valve associated with the specimen represented by figure la. U.S. Naw. 
Mus. Cat. No. 35241g. 
li. Enlargement of the area shown in figure lh. U.S. Nat. Mus. Cat. No. 35241¢. 
1j. Ventral valve tentatively referred to this species, from Locality 88a, Middle Cambrian limestone at Dead- 
wood, Black Hills, South Dakota. U.S. Nat. Mus. Cat. No. 51784a. 
Opotus (WESTONIA) sTONEANUS (Whitfield) (p. 465). 
[Pl. XLIX, figs. 2 and 2a.] 
Figure 2. Cast of the interior of a compressed ventral valve. U.S. Nat. Mus. Cat. No. 51727a. 
2a. Cast of the interior of a ventral valve, showing a few traces of the interior markings of the shell. U.S. 
Nat. Mus. Cat. No. 51727b. 
2b. Enlargement of a cast of the area of a ventral valve. U.S. Nat. Mus. Cat. No. 51727c. 
2c. Cast of the posterior portion of the interior of a ventral valve. U.S. Nat. Mus. Cat. No. 51727d. 
2d. Enlargement of a portion of the outer surface of a ventral valve. U.S. Nat. Mus. Cat. No. 51727e. 
2e. Cast of the interior of a dorsal valve. U.S. Nat. Mus. Cat. No. 51727f. 
2f. Posterior half of a cast of the interior of a dorsal valve. U.S. Nat. Mus. Cat. No. 51727g. 
2e. Cast of the outer surface of a dorsal valve. U.S. Nat. Mus. Cat. No. 51727h. 
The specimens represented are all from Locality 85s, Upper Cambrian sandstone at Prairie du Sac, Sauk County, 
Wisconsin. 
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