PLATE IX. 
a. Central lateral space of area. j. Anterior lateral muscle scar. 
a’. Outer lateral space of area. p. Pedicle groove. 
e. Trapezoidal area, including central, middle ps. Parietal band. 
lateral, and outside lateral muscle scars. y. Visceral cavity. 
{. Flexure line of area. vs. Main vascular sinus. 
h. Central muscle sear. x. Heart-shaped cavity. 
OBoLus RHEA Walcott (p. 410). 
Figure 1. Cast of the interior of a ventral valve, the type specimen, from Locality 98, Upper Cambrian “St. Croix 
sandstone,’ Eau Claire, Wisconsin, which shows the median, pestle-shaped ridge that filled the 
corresponding groove on the interior of the shell. Casts of the lines of growth and the visceral area 
are also finely preserved. U.S. Nat. Mus. Cat. No. 27300a. 
la. Cast of the interior of a ventral valve from Locality 98, Upper Cambrian ‘“‘St. Croix sandstone,”? Eau 
Claire, Wisconsin, in which some of the details of the anterior part of the visceral cavity are more 
clearly defined than in the specimen represented by figure 1. U.S. Nat. Mus. Cat. No. 27300b. 
lb. A dorsal valve, associated on the same hand specimen with the specimen represented by figure la, from 
which the shell has been almost wholly exfoliated. U.S. Nat. Mus. Cat. No. 27300c. 
The specimensrepresented in figures la and 1b are on the same slab of sandstone as the two specimens of Obolus 
namouna represented by figures 2 and 2a, Plate VIII. 
Figure lc. Cast of the interior of a dorsal valve from Locality 84, a Middle Cambrian horizon in the “‘St. Croix sand- 
stone,”? Dresbach, Minnesota, showing the area and traces of the vascular markings. U.S. Nat. 
Mus. Cat. No. 52430a. 
OBOLUS REFULGENS Matthew (p. 409). 
Ficure 2. Cast of the interior of a ventral valve, the type specimen, that is flattened in the shale. University of 
Toronto, Canada. Redrawn from the specimen figured by Matthew [1892, Pl. XII, fig. 6d (ventral)] 
as Obolus refulgens; and copied by him [1903, Pl. XI, fig. 4a] as Monobolina refulgens. 
2a. Cast of the interior of a dorsal valve that preserves the original convexity. University of Toronto, Canada. 
Redrawn from the specimen figured by Matthew [1892, Pl. XII, fig. 6d (dorsal)] as Obolus refulgens; 
and copied by him [1903, Pl. XI, fig. 4b] as Monobolina refulgens. 
2b. Veniral valve flattened in the shale and compressed laterally so as to give it more the outline of Lingulella 
than of Obolus. University of Toronto, Canada. 
2c and 2d. Dorsal valves flattened in the shale and slightly distorted. University of Toronto, Canada. 
The specimens represented all occur in Locality 308, Upper Cambrian, Division 3c of Matthew, Navy Island, 
St. John Harbor, New Brunswick. Specimens represented by figures 2 and 2a occur in an arenaceous shale which 
preserves the specimens in better condition than in the argillaceous shales. 
OpoLtus PANDEMIA Walcott (p. 407). 
Figure 3. Exterior of a slightly flattened and distorted ventral valve. U.S. Nat. Mus. Cat. No. 27301a. 
3a. Cast of the interior of a ventral valve, the type specimen, that has been compressed in its-posterior portion 
so as to force the visceral cavity back nearly to the area. U.S. Nat. Mus. Cat. No. 27301b. 
3b. Interior of the ventral valve showing an elevated pestle-shaped median ridge that occupies the place of 
. the heart-shaped cavity of most species of Obolus. U.S. Nat. Mus. Cat. No. 27301c. 
3c. Cast of the interior of a dorsal valve, showing the area. U.S. Nat. Mus. Cat. No. 27301d. 
3d. Cast showing part of the interior of a dorsal valve, showing traces of the central muscle scars, and the 
thick lamination of the shell toward the margin. U.S. Nat. Mus. Cat. No. 27301e. 
The specimens represented are all from Locality 10a, Middle Cambrian, Bays Mountains, Knoxville, Tennessee. 
OxsoLus LOPERI Walcott (p. 395). 
Figure 4. Imperfect ventral valve from Locality 335a, Cambro-Ordovician sandstone, Cement Creek, 3 miles north 
of Hot Springs, Ouray County, Colorado, showing the general form and the several layers forming 
the shell. U.S. Nat. Mus. Cat. No. 27303a. 
4a. Partly exfoliated dorsal valve, the type specimen, associated with the specimen represented by figure 4. 
U.S. Nat. Mus. Cat. No. 27303b. 
34 
