DESCRIPTION OF GENERA AND SPECIES. 67 
inner layers by radiating and concentric strie; the outer surface has numerous fine, 
elevated, slightly irregular, concentric strie that, with a strong lens, give it a rough- 
ened appearance. 
An interior of a dorsal valveshows a well-developed area, the greater portion of 
which is taken by the broad pedicle groove; a deep umbonal cavity with a narrow 
median septum and obscure main vascular sinuses on the outer margins of the cavity. 
Outside the sinuses, on the postero-lateral slope, relatively large muscle-scars are 
indicated. 
With the data available no satisfactory comparisons can be made with other 
species. Obolus obscurus is a large, strong shell of the general type of Obolus rhea 
Walcott [1912b, plate rx, figs. 1, 1a—c] of the Middle Cambrian of Wisconsin. 
Since the publication of the original description of this species in 1905 better 
specimens referred to this species were received from the Middle Cambrian of 
Shan-si. These illustrate the form and convexity of the central valve, and show it 
to be similar to that of Obolus chinensis [p. 64]. The concentric ridges are more nearly 
perfect than those on the fragments associated with the type specimens from Shan- 
tung. The concentric strie or ridges are elevated, rounded, with sharp depressions 
between them; the ridges are little more than elevated strie at the aperture, becom- 
ing gradually coarser over the central and anterior portions of the valve, where there 
are from five to seven ridges in a distance of 1 mm.; toward the apex there are more 
than double the number in the same distance. 
Obolus obscurus is a larger and more radiate shell than O. chinensis, and is also 
distinguished by the strong, concentrically striated surface, which resembles that of 
Micromitra (Paterina) labradorica orientalis [p. 62], from which it differs in the form 
of the shell. It differs from O. (Acritis) antiquissimus (Eichwald) [1843, plate 142; 
Walcott, 1912), plate xu, fig. 3, and plate xv, figs. 2, 2a—d]in having the concentric 
strie or ridges more regular, and very rarely bifurcating. Obolus (A. ?) rugatus 
Walcott [idem, plate x11, fig. 2] has much stronger concentric ridges. The surface 
of O. nundina Walcott [1905a, p. 326] is marked by irregularly spaced, rounded, 
concentric ridges, with rounded depressions between them instead of the V-shaped 
depressions and regularly spaced ridges of O. obscurus. 
Formation and Locality.—Middle Cambrian: (C63) Sandy shale near the base 
of the Kiu-lung group [Blackwelder, 1907a, p. 37 (third paragraph), and fig. 8a (bed 
32), p- 29], 3.5 miles (5.6 km.) southwest of Yen-chuang, Sin-t’ai district, Shan-tung; 
also (C75) limestone near the base of the Ki-chéu formation [Willis and Black- 
welder, 1907, p. 143], 4.5 miles (7.2 km.) south of Wu-t’ai-hién, Shan-si, China. 
Collected by Eliot Blackwelder. 
Obolus shansiensis Walcott. 
Plate 2, Figures 4, 4a—g. 
Obolus shensiensis WALCOTT, 1905, Proc. U.S. Nat. Mus., vol. xxviul, p.327. (Described and discussed 
essentially as below as a new species, the spelling ‘“‘shensiensis’’ being an error, because the type 
specimens came from a locality in the Province Shan-si.) 
Obolus shansiensis WaLcorT, 1912, Monogr. U.S. Geol. Survey, vol. 1, p. 415, plate x1, figs. 7, 7a-c. 
(A copy of the preceding reference.) 
General form ovate, with the ventral valve broadly subacuminate and dorsal 
valve obtusely rounded. Valves moderately convex in the specimens embedded in 
a fine-grained, dark limestone. Surface marked by fine, sharp, concentric striz and 
traces of irregular, obscurely defined, low, radiating ridges; the interior layers show 
fine, radiating strie and concentric lines. The shell is strong and built up of numer- 
ous lamelle oblique to the outer surface. The largest ventral valve has a length of 
gmm., width 7mm. Nothing is known of the interior of the valves. 
