DESCRIPTION OF PLATE 4. 
Acrotreta pacifica Walcott (p. 75). 
1 and 1a. Summit and side views of ventral valve, X 15. From Locality C5, Middle Cambrian, Kiu- 
lung group, southwest of Yen-chuang, Shan-tung. U.S. Nat. Mus., Cat. No. 52506a. 
1b, 1c, and 1d. Summit, back, and side views of a small ventral valve, X 15. From same locality as 
fiz. 1. U.S. Nat. Mus., Cat. No. 525060. 
1e. Partly exfoliated dorsal valve, X 15. From Locality C2, Middle Cambrian, Kiu-lung group, 
south of Yen-chuang, Shan-tung. U.S. Nat. Mus., Cat. No. 52508. 
1f. Summit view of a ventral valve crushed down in a fine arenaceous shale, X 8. Locality 35p, Middle 
Cambrian, Fu-chéu series, Liau-tung, Manchuria. U.S. Nat. Mus., Cat. No. 57764. 
Acrotreta shantungensis Walcott (p. 76). 
2, 2a, and 2b. Summit, side, and back views of ventral valve, X 15. U.S.Nat. Mus., Cat. No. 52512a. 
2c. Cast of interior of a ventral valve, showing cast of apical callosity, foraminal aperture, and base of 
main vascular sinuses, X 15. U.S. Nat. Mus., Cat. No. 525120. 
2d. Exterior of a dorsal valve, X 15. U.S. Nat. Mus., Cat. No. 52512¢. 
2e. Interior of a dorsal valve, X 15. U.S. Nat. Mus., Cat. No. 52512d. 
The specimens illustrated are from Locality C62, Middle Cambrian, Kiu-lung group, argillaceous lime- 
stone, south of Yen-chuang, Shan-tung. 
Discinopsis sulcatus (Walcott) (p. 79). 
3. Cast of interior of a ventral valve—the type specimen— X 8. From Locality C56, Upper Cambrian, 
Ch’au-mi-tién limestone, west of T'si-nan, Shan-tung. U.S. Nat. Mus., Cat. No. 52522. 
Billingsella pumpellyi Walcott (p. 79). 
4. Summit and side views (X 3) of a ventral valve of a specimen from C61, Upper Cambrian, Kiu-lung 
group, southwest of Yen-chuang, Shan-tung. U.S. Nat. Mus., Cat. No. 52558qa. 
4a. Cast of interior of a ventral valve, showing tripartite divisions of the umbonal cavity, and the strong 
main vascular sinuses, X 3. U.S. Nat. Mus., Cat. No. 52557a. 
4b. Partly exfoliated dorsal valve, X 3. U.S. Nat. Mus., Cat. No. 525570. 
4c. Enlargement of the radiating coste toward the front margin of the shell. U.S. Nat. Mus., Cat. 
No. 525580. 
Figures 4a-c are of specimens from Locality C386, Upper Cambrian, Ch’au-mi-tién limestone, at 
Ch’au-mi-tién, Shan-tung. 
Billingsella richthofeni Walcott (p. 80). 
5. Summit view and side outline of a ventral valve, X 4. From Locality C3, Lower Cambrian, Man-t’o 
shale, southwest of Yen-chuang, Shan-tung. U.S. Nat. Mus., Cat. No. 52559. 
Eoorthis agreste (Walcott) (p. 81). 
6. Summit and side views of a ventral valve, X 2. U.S. Nat. Mus., Cat. No. 52545a. 
6a. Enlargement of radiating costee near front margin, X 3. U.S. Nat. Mus., Cat. No. 525450. 
From Locality C28, Middle Cambrian, Ch’ang-hia limestone, east-southeast of Ch’ang-hia, Shan-tung. 
Eoorthis doris (Walcott) (p. 81). 
7. Summit and side views of a ventral valve, X 4. U.S. Nat. Mus., Cat. No. 52551a. 
7a. Summit and side views of a dorsal valve which is nearly as convex as the ventral valve, X 4. U.S. 
Nat. Mus., Cat. No. 525510. 
From Locality C64, Upper Cambrian, Kiu-lung group, southwest of Yen-chuang, Shan-tung. 
Eoorthis kayseri (Walcott) (p. 82). 
8. Slightly distorted ventral valve. Natural size. U.S. Nat. Mus., Cat. No. 52546a. 
8a. Small, partly exfoliated dorsal valve, X 3. U.S. Nat. Mus., Cat. No. 525460. 
8b. Cast of interior of a dorsal valve, showing cast of strong median ridge and vascular sinuses, X 2. 
U.S. Nat. Mus., Cat. No. 52546c. : 
The three specimens illustrated are from Locality C64, Upper Cambrian, Kiu-lung group, southwest 
of Yen-chuang, Shan-tung. 
Eoorthis sp. undt. (6) (Walcott) (p. 85). 
g. Exterior of a partly abraded ventral valve, X 3. From Locality C71, Middle Cambrian, Ki-chéu 
limestone, southwest of Tung-yii, Shan-si. U.S. Nat. Mus., Cat, No. 52555. 
Eoorthis kichouensis (Walcott) (p. 82). 
10. Exterior of a ventral valve, and side outline, X 2. From Locality C75, Middle Cambrian, Ki-ché6u 
formation, south of Wu-t’ai-hién, Shan-si. U.S. Nat. Mus., Cat. No. 52550. 
Eoorthis linnarssoni (Kayser) (p. 83). 
11. Exterior of a partly exfoliated ventral valve, X 2.5. From Locality C64, Upper Cambrian, Kiu- 
lung group, southwest of Yen-chuang, Shan-tung. U.S. Nat. Mus., Cat. No. 52549. 
11a. Copy of portion of original figure from Kayser [1883, pl. m1, fig. 1]. 
Eoorthis pagoda (Walcott) (p. 83). 
12 and 12a, Exterior views of partly exfoliated ventral valves, X 2. U.S. Nat. Mus., Cat. Nos. 
525520 and 525520. 
12b. Partly exfoliated dorsal valve, X 2. U.S. Nat. Mus., Cat. No. 52552¢. 
12¢. ee of a cee valve that has been slightly injured by abrasion, X 4. U.S. Nat. Mus., Cat. 
0. 52552d. 
12d. Enlargement of the cast of a portion of the outer surface, X 6. U.S. Nat. Mus., Cat. No. 52552e. 
All of the specimens illustrated are from Locality C56, Upper Cambrian, Ch’au-mi-tién limestone, west 
of Tsi-nan, Shan-tung. 
Eoorthis sp. undt. (a) (Walcott) (p. 84). 
13. Summit and side views of a young shell at first referred doubtfully to Orthis (Plectorthis), X 8. 
From Locality C26, Middle Cambrian, Ch’ang-hia limestone, north-northeast of Ch’ang-hia, 
Shan-tung. U.S. Nat. Mus., Cat. No. 52554. 
Huenella orientalis (Walcott) (p. 85). 
14. Exterior of ventral valve, with side outline, X 2.5. U.S. Nat. Mus., Cat. No. 52564a. 
14a. Partly exfoliated dorsal valve, with side outline, X 2.5. U.S. Nat. Mus., Cat. No. 525640. 
14), Smooth form of a dorsal valve, preserving a portion of the outer surface of the shell, X 3. U.S. 
Nat. Mus., Cat. No. 52564c. 
From Locality C64, Upper Cambrian, Kiu-lung group, southwest of Yen-chuang, Shan-tung. 
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