PLATH X. 
c, Trapezoidal area, including central, middle s. Median septum. 
lateral, and outside lateral muscle sears. v. Visceral cavity. 
h. Central muscle scar. vs. Main vascular sinus. 
j. Anterior lateral muscle scar. x. Heart-shaped cavity. 
OxpoLus mioKwirzi Walcott (p. 403). 
Traure 1. Cast of asmall ventral valve, the type specimen, showing the deeply impressed vascular and visceral mark- 
ings. It will be noted that the visceral cavity extends far forward. U.S. Nat. Mus. Cat. No. 27299a. 
la. Cast of the interior of a ventral valve in which the visceral cavity is much larger proportionally than that 
represented in figure 1, and in which the areas occupied by the central, middle lateral, and outside 
lateral scars are unusually large and prominent. More than two-thirds of the entire length of the 
shell is taken up by the visceral area. U.S. Nat. Mus. Cat. No. 27299b. 
1b. Portion of the cast of a small ventral valve in which the interior markings have been still more deeply 
impressed than in the specimens shown by figures 1 and la. The heart-shaped cavity (x), the trape- 
zoidal area (c), and the main vascular sinus (vs) must have been placed upon a strongly built up 
platform in the interior of the shell. U.S. Nat. Mus. Cat. No. 27299c. 
le. Cast of the interior of a large ventral valve in which the visceral area is limited to the posterior half of the 
shell. U.S. Nat. Mus. Cat. No. 27299d. 
1d. Cast of the interior of a large dorsal valve which shows the relative position of the central (h) and anterior 
lateral (j) muscle scars, and the form of the visceral cavity. U.S. Nat. Mus. Cat. No. 27299e. 
le. Partly exfoliated: dorsal valve which illustrates the lamellated character of the sheil and the strong 
concentric undulations of growth which appear on some specimens. U. S. Nat. Mus. Cat. No. 
272998. 
1f. Cast of a large dorsal valve in which the visceral area is mainly upon the posterior half, as in the ventral 
valve represented by figure le. U.S. Nat. Mus. Cat. No. 27299¢. 
lg. Cast of the interior of a small dorsal valve in which the visceral area occupies the central portion of the 
shell. U.S. Nat. Mus. Cat. No. 27299h. 
th. Cast of the interior of a dorsal valve showing a remarkable development of the area about the central 
muscle scars. U.S. Nat. Mus. Cat. No. 27299i. 
li. Cast of the interior of a ventral valve in which the main vascular sinus and other interior markings of 
the shell are less deeply impressed than in specimens represented by figures la and 1b. 
The specimens represented in figures 1, la-i are from Locality 79, Upper Cambrian sandstone, at Hudson, Wis- 
consin. 
Frieure 1j. Cast of a ventral valve which is somewhat doubtfully referred to this species, owing to its not showing the 
same strong interior markings. This, however, may be the result of the condition of preservation. 
U.S. Nat. Mus. Cat. No. 56937a. 
1k. Partly exfoliated dorsal valve associated with the specimen represented by figure 1j. It is probable 
that a larger collection of material would show that figures 1j and 1k represent a distinct species. 
U.S. Nat. Mus. Cat. No. 56937b. 
The specimens represented in figures 1j and 1k are from Locality 79b, Upper Cambrian sandstone at Hudson, 
Wisconsin, a slightly different horizon than the one containing the specimens represented in figures 1, la-i. 
Opotus M&RA (Hall and Whitfield) (p. 399). 
Ficure 2. Partly exfoliated and broken ventral valve from’ Locality 313j, Upper Cambrian limestone, Eureka 
district, Nevada. U.S. Nat. Mus. Cat. No. 24552a. 
2a. Partly exfoliated dorsal valve associated with the specimen represented by figure 2. U.S. Nat. Mus. 
Cat. No. 24552b. Type specimen figured by Hall and Whitfield [1877, Pl. I, fig. 6]as Lingulepis mera. 
2b. More élongated, partly exfoliated dorsal valve from Locality 313j, Upper Cambrian limestone, Eureka 
district, Nevada. U. S. Nat. Mus. Cat. No. 24552c. Specimen figured by Hall and Whitfield 
(1877, Pl. I, fig. 5) as Lingulepis mexra. 
2c. Crushed and partly exfoliated ventral valve, doubtfully referred to this species, from Locality 205, 
limestone forming passage beds between the Upper Cambrian and the Ordovician, on Roundtop 
Mountain, Eureka district, Nevada. U.S. Nat. Mus. Cat. No. 51562a. 
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