66 RESEARCH IN CHINA. 



Ohnlus mathmlis (Hall)? WALCOTT, 1905, Proc. U. S. Nat. Mus., vol. xxvin, p. 325. (Mentioned as 



below. Doubtfully identifies the species from China.) 

 Obolus matinalis (Hall)? WALCOTT, 1912, Monogr. U. S. Geol. Survey, vol. u, p. 402, plate vni, figs. 



3 and 30. (Copy of preceding reference.) 



A form indistinguishable from this species occurs in a gray limestone of Upper 

 Cambrian age. Only the general form of partly exfoliated shells is known. 



Formation and Locality. Middle Cambrian: ((.'54) Lower part of Ch'au-mi- 

 tien limestone [Blackwelder, 19070, p. 42 (part of last list of fossils)], near top of 

 limestone knoll two-thirds of a mile (i.i km.) west of Tsi-nan, Shan-tung, China. 



Collected by Eliot Blackwelder. 



Fragments of a similar shell were found in the (C67) stream gravels used in 

 making the railroad grade west of the west city gate at Tsi-nan. 



Collected by Eliot Blackwelder. 



Obolus minimus Walcott. 

 Plate 2, Figures 3, 30. 



Obolus minimus WALCOTT, 1905, Proc. U. S. Nat. Mus., vol. xxvin, p. 325. (Described as below as a 



new species.) 

 Obolus minimus WALCOTT, 1912, Monogr. U.S. Geol. Survey, vol. LI, pp. 404-405, plate xi, figs. 8 and 80. 



(Copy of the preceding reference.) 



This is a small shell of the general form of Obolus shansiensis [p. 67]. The 

 ventral valve is obtusely acuminate and the dorsal nearly circular; valves gently 

 convex. Surface marked by rather strong, concentric lines of growth and numerous 

 very fine, concentric striae. The inner layers of shell are shiny black and orna- 

 mented with numerous fine, radiating striae and concentric lines. Shell built up of 

 several thin layers or lamellae that form a shell which is thin over the umbonal region 

 and gradually thickens toward the front and side margins, as the short oblique 

 lamellae become more numerous. The three specimens in the collection average 3 

 mm. in transverse diameter; the ventral is a little longer than the dorsal valve. 



A partly exfoliated ventral valve shows a well-marked visceral area, extending 

 forward about one-third of the length of the shell ; also narrow main vascular sinuses 

 starting near the apex and extending obliquely forward well into the valve, about 

 midway between the median line of the valve and the lateral margins. 



This neat little shell is distinguished by its nearly circular outline, low con- 

 vexity, and small size. 



Formation and Locality. Middle Cambrian: (Cl) Lower shale member of 

 the Kiu-lung group [Blackwelder, 19070, pp. 37 and 40 (part of the third list of 

 fossils), and fig. 10 (bed 4), p. 38], 2 miles (3.2 km.) south of Yen-chuang, Sin-t'ai 

 district, Shan-tung, China. 



Collected by Eliot Blackwelder. 



Obolus obscurus Walcott. 

 Plate i, Figures 12, i2a-d. 



Obolus obscurus WALCOTT, 1905, Proc. U. S. Nat. Mus., vol. xxvin, p. 326. (Described and discussed 



essentially as in the first two paragraphs below as a new species.) 

 Obolus obscurus WALCOTT, 1912, Monogr. U.S. Geol. Survey, vol. LI, pp. 406-407, plate xi, figs. 9, ga-d. 



(Described and discussed essentially as below.) 



This species is represented by one interior of a small dorsal valve which is very 

 distinctive, the exterior of a crushed dorsal valve, and numerous fragments of the 

 shell scattered through the hard, dark gray, shaly sandstone. The large dorsal 

 valve has a length of 9 mm., width 7 mm. The shell is strong and marked on its 



