DESCRIPTION OF GENERA AND SPECIES. 77 



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 striae and lines of growth. 



This shell in size and general form is closely related to Acrotrcta microscopica 

 (Shumard) [Walcott, 191 2b, plate LXVII, figs, i, la-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: (062) Earthy layer in the middle 

 limestone of the Kiu-lung group [Blackwelder, 19070, pp. 37 and 40 (last list of fos- 

 sils), and fig. io(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-chou 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-hien, Shan-si, China; also (071), Middle Cambrian, massive cliff-making lime- 

 stone in the central portion of the Ki-chou 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 (032) a fine-grained bluish-black limestone bowlder believed to have 

 come from the lower part of the Ki-sin-ling limestone [Blackwelder, 1907*:, p. 272], 

 collected in river drift i mile ( 1.6 km.) south of Chon-p'ing-hien, on the Nan-kiang 

 River, southern Shen-si, China. 



This species is somewhat doubtfully identified from the Middle Cambrian of 

 Locality 01, 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 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 richthojeni zone in association with Obolus 

 chinensis at the following localities. 



Formation and Locality. Middle Cambrian: (35n and 35 r) Fu-ch6u series; 

 limestones near the base of the series just above the white quartzite [see Blackwelder, 

 19076, p. 92, for general section giving stratigraphic relations], collected in a low bluff 

 on the shore of Tschang-hsing-tau Island, and (35 o and 3fig) shales about 130 feet 

 (40 m.) above the white quartzite [see Blackwelder, 19076, p. 92, for general section 

 giving stratigraphic relations], collected in drainage cuts a short distance back from 

 the bluff [see 35 n] forming the shore of Tschang-hsing-tau Island, and (36 h) shales 

 about 130 feet (40 m.) above the white quartzite [idem, 19076, 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, loa-b. 



Acrotreta venia WALCOTT, 1911, Smithsonian Misc. Coll., vol. 57, No. 4, p. 75, plate 14, figs. 4 and 40. 

 (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- 



