Page:Walcott Cambrian Geology and Paleontology II.djvu/131

NO. 4 Interiors of the ventral valve indicate that the area was relatively shorter than that of L. similis (Walcott). Most of the shells in the limestone are less than 2 mm. in length; a few ventral valves in the shale are 3 to 5 mm. long.

Formation and locality.—Middle Cambrian: (35q and 36h) Fu-chóu series; abont 200 feet (61 m.) above the white quartzite; also in shales about 130 feet (40 m.) above the white quartzite; 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.

A form flattened in the shale of locality 36f, which is much higher, but in the same section as the locality represented by L. marcia, has the outline of the latter species and is tentatively referred to it. This gives an extended stratigraphic range for the species, but not so great as that of L. similis (Walcott) which occurs in both the Middle and Upper Cambrian.

Acrotreta venia is closely related to A. shantungensis Walcott. It differs in having a slightly less elevated ventral valve and a more distinct and broader flattening of the posterior side and margin. The faintly defined false area and apex of the valve curve slightly over the porterior margin. The dorsal valve does not show the median depression of the dorsal valve of A. shantungensis. Of the American Pacific Province species of Acrotreta, it most nearly approaches A. idahoensis (Walcott) in the external form of the valves. The vascular markings on the interior of the dorsal valve differ in detail as may be seen by comparing them.

Formation and locality.—Middle Cambrian: (35q) Fu-chóu series; about 200 feet (61 m.) above the white quartzite, 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.

Form an elongate, slender, rounded tube, with the dorsal face less convex than the sides and ventral face. Transverse section round,