DESCRIPTION OF GENERA AND SPECIES. 67 



inner layers by radiating and concentric striae; the outer surface has numerous fine, 

 elevated, slightly irregular, concentric striae that, with a strong lens, give it a rough- 

 ened appearance. 



An interior of a dorsal valve shows 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 [19126, plate ix, figs, i, xo-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 striae or ridges are elevated, rounded, with sharp depressions 

 between them ; the ridges are little more than elevated striae 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 i 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. cliiiicnsis, 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 0. (Acritis) antiquissimus (Eichwald) [1843, plate 142; 

 Walcott, 19126, plate xin, fig. 3, and plate xv, figs. 2, 2a-d] in having the concentric 

 striae or ridges more regular, and very rarely bifurcating. Obolus (A. ?) rugatus 

 Walcott [idem, plate xin, fig. 2] has much stronger concentric ridges. The surface 

 of 0. nundina Walcott [1905(2, 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 0. obscurus. 



Formation and Locality. Middle Cambrian: (063) Sandy shale near the base 

 of the Kin-lung group [Blackwelder, 19070, 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 (075) limestone near the base of the Ki-chou formation [Willis and Black- 

 welder, 1907, p. 143], 4.5 miles (7.2 km.) south of Wu-t'ai-hien, Shan-si, China. 



Collected by Eliot Blackwelder. 



Obolus shansiensis Walcott. 

 Plate 2, Figures 4, 43 g. 



Obolus shensiensis WALCOTT, 1905, Proc. U.S. Nat. Mus.,vol. xxvm, p. 32 7. (Described and discussed 

 essentially as below as a new species, the spelling "shcnsiensis" being an error, because the type 

 specimens came from a locality in the Province Shan-si.) 



Obolus shansiensis WALCOTT, 1912, Monogr. U.S. Geol. Survey, vol. LI, p. 415, plate xi, figs. 7, ^a-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 striae and 

 traces of irregular, obscurely defined, low, radiating ridges; the interior layers show 

 fine, radiating striae and concentric lines. The shell is strong and built up of numer- 

 ous lamellae oblique to the outer surface. The largest ventral valve has a length of 

 9 mm., width 7 mm. Nothing is known of the interior of the valves. 



