90 RESEARCH IN CHINA. 



regularly incurved, with the plane of the coiling toward the right dorsal side. The 

 section of the outer whorl near the aperture is elongate oval, with the left ventral 

 side somewhat flattened. The outer whorl widens rapidly toward the aperture, 

 especially on the right ventral (inner) side, which gives the outline, when looked at 

 from the dorsal ridge, an oblique, unsymmetrical appearance. 



Surface marked by concentric lines of growth which arch backward upon the 

 dorsum, indicating a dorsal sinuosity in the peristome. 



Greatest diameter of the shell, 2.5 mm.; dorso-ventral diameter of whorl near 

 the aperture, 1.75 mm.; greatest lateral diameter not measurable, but apparently 

 not more than one-half the antero-posterior diameter. 



This species differs from Pelagiella chronus [p. 88] in the size of the outer whorl, 

 minute inner whorl, and the absence of longitudinal ridges, features which also 

 distinguish it from Platyceras primczvum Billings [1871, p. 220] and allied forms. 



Formation and Locality. Upper Cambrian: (C56) Lower part of Ch'au-mi- 

 tien limestone, 25 feet (7.5 m.) below the top of Pagoda Hill [Blackwelder, 19070, 

 p. 42 (part of last list of fossils)], i mile (1.6 km.) west of Tsi-nan, Shan-tung, China. 



Collected by Eliot Blackwelder and Li San. 



Pelagiella willisi (Walcott). 

 Plate 5, Figures 12, 13. 



Platyceras willisi WALCOTT, 1906, Proc. U. S. Nat. Mus., vol. xxx, p. 572. (Described and discussed as 

 a new species essentially as below.) 



Shell minute, consisting of two whorls somewhat irregularly incurved; the apex 

 and one side of the aperture lie in the same plane; the outer whorl increases in 

 size gradually through its first half, and then expands more rapidly toward the 

 aperture; a cross-section of the outer whorl shows the dorsal (outer) side to be 

 gently convex and the inner side somewhat more strongly convex, a rounded dorsal 

 angle being formed where the two sides unite on the outer edge. 



The surface is marked by concentric lines of growth parallel to the aperture. 



The greatest diameter of the largest shell is 1.6 mm. 



This species differs from Pelagiella chronus [p. 88] in having a more slender, 

 rounded outer whorl, without trace of the dorsal ridge characteristic of that species. 

 It differs from P. clytia [p. 89] in being coiled on the plane of the dorsal side instead 

 of on the plane of the median line. 



Formation and Locality. Middle Cambrian: (C72) Thin green-gray limestone 

 interbedded with ocherous and green clay shales, overlying the massive oolite in the 

 Ki-chou formation [Willis and Blackwelder, 1907, pp. 139 and 145 (third list of 

 fossils)], 4 miles (6.4 km.) east of Fang-lan-chon, Shan-si, China. 



Collected by Eliot Blackwelder. 



Genus HELCIONELLA Grabau and Shimer. 

 Helcionella GRABAU and SHIMER, 1909, North American Index Fossils, vol. in, p. 607. 



Helcionella ? clurius (Walcott). 

 Plate 5, Figure 7. 



Stenotheca clurius WALCOTT, 1905, Proc. U. S. Nat. Mus., vol. xxix, p. i.s. (Described and discussed 

 as a new species essentially as below.) 



This form is represented by the interior cast of a large, slender, slightly curving 

 shell. The cast has a length of 16 mm., with a diameter where it is broken off at 



