PLATE VIII. 
a. Central lateral space of area. o’. Overlapping of lines of growth of area. 
a’. Outer lateral space of area. p. Pedicle groove. 
ce. Trapezoidal area, including central, middle pvs. Peripheral branches of the vascular system. 
lateral, and outside lateral muscle scars. ps. Parietal band. 
f. Flexure line of area. s. Median septum. 
h. Central muscle scar. v. Visceral cavity. 
i. Transmedian muscle scar. vs. Main vascular sinus. 
j. Anterior lateral muscle scar. ivs. Lateral branches of vascular system. 
k. Middle lateral muscle scar. x. Heart-shaped cavity. 
1. Outside lateral muscle scar. 
OBOLUS MATINALIS (Hall) (p. 400). 
I'ieure 1. Exterior of an almost perfect ventral valve from Locality 328e, a Middle Cambrian horizon in the ‘‘St. Croix 
sandstone,” St. Croix Falls, Wisconsin. U.S. Nat. Mus. Cat. No. 51564a. 
la. Large, partly exfoliated ventral valve from Locality 67¢, Upper Cambrian sandstone, Tatur Hill, Burnet 
County, Texas, showing the radial striation of the inner layers of the shell. U.S. Nat. Mus. Cat. No. * 
52420a. 
Jb. Partly exfoliated ventral. valve from Locality 67, Upper Cambrian sandstone, Tatur Hill, Burnet County, 
Texas: U.S. Nat. Mus. Cat. No. 52419a. 
le. Partly exfoliated ventral valve associated with the specimen represented by figure 1. U.S. Nat. Mus. 
Cat. No. 51564b. 
1d. Cast of the interior of a ventral valve showing the details of the interior of the shell with unusual clear- 
ness, from Locality 82b, Upper Cambrian “‘St. Croix sandstone,’”’ Taylors Falls, Chisago County, 
Minnesota. U.S. Nat. Mus. Cat. No. 51563a. 
le. Cast of the interior of a ventral valve associated with the valve represented by figure 1. U.S. Nat. Mus. 
Cat. No. 51564c. y 
lf. Interior of a broken ventral valve from Locality 339d, Upper Cambrian ‘‘St. Croix sandstone” at Taylors 
Falls, Minnesota, showing the thickened shell under the visceral area. U.S. Nat. Mus. Cat. No. 
51563c¢. 
lg. Interior of a ventral valve, in which there is no special thickening beneath the visceral area, associated 
with the specimen represented by figure lf. U.S. Nat. Mus. Cat. No. 51563d. 
th. Cast of the interior of a dorsal valve associated with the specimen represented by figure 1d. U.S. Nat. 
Mus. Cat. No. 51563b. 
li. Cast of the interior of a dorsal valve associated with the valve represented by figures 1, lc, and le. U.S. 
Nat. Mus. Cat. No. 51564d. 
1j. Interior of a ventral valve in which there has been a considerable thickening of the shell so as to form a 
platform to which the muscles and visceral parts were attached. From Locality 339d, Upper Cam- 
brian “‘St. Croix sandstone,” Taylors Falls, Minnesota. U.S. Nat. Mus. Cat. No. 51563e. 
1k. Partly exfoliated dorsal valve from Locality 71, Upper Cambrien limestone, Cold Creek Canyon, Burnet 
County, Texas. U.S. Nat. Mus. Cat. No. 51566a, 
OBOLUS TETONENSIS NINUS Walcott (p. 418). 
[Pl. XI, figs. 1, la-g.] 
Figure 11. Ventral valve, showing the strongly marked radiating strie of the inner layers. 
1m. Cast of the interior of a ventral valve. , 
In. Cast of the interior of a dorsal valve. 
lo. Cast of the interior of a partly exfoliated dorsal valve. 
The specimens represented by figures 11 to lo are from Locality 68, Upper Cambrian sandstone, Packsaddle Moun- 
fain, Llano County, Texas. U.S. Nat. Mus. Cat. Nos. 51644a-d, respectively. 
OsBoLus NAMoUNA Walcott (p. 406). 
Figure 2. Cast of the interior of a ventral valve, the type specimen, associated on the same hand specimen with 
the shells of Obolus rhea represented by figures la and 1b, Plate IX, from Locality 98, Upper Cambrian 
“St. Croix sandstone,” Eau Claire, Wisconsin. U.S. Nat. Mus. Cat. No. 27304a. 
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