DESCRIPTION OF GENERA AND SPECIES. 79 
terior margin; main vascular sinuses rather strong; they start beneath the apex and 
extend forward a short distance from the outer lateral margin of the shell. Surface 
marked by fine, concentric striz and lines of growth. 
This shell in size and general form is closely related to Acrotreta microscopica 
(Shumard) [Walcott, 1912, plate Lxvu, figs. 1, 1a—h, 10, 2, 2a-d] of the Middle Cam- 
brian fauna of the United States. It occurs abundantly in a chocolate-colored lime- 
stone associated with Obolus chinensis [p. 64]. 
Formation and Locality.—Middle Cambrian: (€62) Earthy layer in the middle 
limestone of the Kiu-lung group [Blackwelder, 1907a, pp. 37 and 40 (last list of fos- 
sils), and fig. 10 (base of bed 7), p. 38], 2.5 miles (4 km.) south of Yen-chuang, on the 
north-northeast spur of Hu-lu-shan, Sin-t’ai district, Shan-tung, and (C37) Upper 
Cambrian, upper part of the Ki-chéu limestone (the fossils from this locality are not 
listed, but the presence of Cambrian strata at the locality is mentioned by Willis and 
Blackwelder [1907, p. 146]), in dense black limestone nodules in green-gray shales 10 
feet (3 m.) below the base of the cliff limestone, 8 miles (12.8 km.) south of Ting- 
hiang-hién, Shan-si, China; also (C71), Middle Cambrian, massive cliff-making lime- 
stone in the central portion of the Ki-chéu formation [Willis and Blackwelder, 1907, 
pp. 139 and 145 (second list of fossils)], 4 miles (6.4 km.) southwest of Tung-yii, 
Shan-si, and (C32) a fine-grained bluish-black limestone bowlder believed to have 
come from the lower part of the Ki-sin-ling limestone [Blackwelder, 1907c, p. 272], 
collected in river drift 1 mile (1.6 km.) south of Chén-p’ing-hién, on the Nan-kiang 
River, southern Shen-si, China. 
This species is somewhat doubtfully identified from the Middle Cambrian of 
Locality C1, lower shale member of the Kiu-lung group [Blackwelder, 1907a, 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 Bailey Willis and Eliot Blackwelder. 
In Manchuria Prof. J. P. Iddings found this species in great abundance both in 
limestones and shales of the Dorypyge richthofeni zone in association with Obolus 
chinensis at the following localities. 
Formation and Locality—Middle Cambrian: (85n and 35r) Fu-chéu series; 
limestones near the base of the series just above the white quartzite [see Blackwelder, 
1907), p. 92, for general section giving stratigraphic relations], collected in a low bluff 
on the shore of Tschang-hsing-tau Island, and (850 and 36g) shales about 130 feet 
(40 m.) above the white quartzite [see Blackwelder, 1907), p. 92, for general section 
giving stratigraphic relations], collected in drainage cuts a short distance back from 
the bluff [see 35n] forming the shore of Tschang-hsing-tau Island, and (36h) shales 
about 130 feet (40 m.) above the white quartzite [idem, 1907), p. 92], collected in a 
low bluff on the shore of Tschang-hsing-tau Island, east of Niang-niang-kung, Liau- 
tung, Manchuria, China. 
Collected by J. P. Iddings and Li San. 
Acrotretra venia Walcott. 
Plate 3, Figures 10, 10a—b. 
Acrotreta venia WALCOTT, 1911, Smithsonian Misc. Coll., vol. 57, No. 4, p. 75, plate 14, figs. 4 and 4a. 
(Described and discussed as a new species essentially as below.) 
Acrotreta venia is closely related to A. shantungensis Walcott [p. 76]. It differs 
in having a slightly less elevated ventral valve and a broader and more distinct 
flattening of the posterior side and margin. The faintly defined false area and apex 
of the valve curve slightly over the posterior margin. The dorsal valve does not 
show the median depression of the dorsal valve of A. shantungensis. Of the Amer- 
