DESCRIPTION OP PLATE 4. 



Acrotreta pacifica Walcott (p. 75). 



I and 10. Summit and side views of ventral valve, X 15. From Locality 05, Middle Cambrian, Kiu- 



lung group, southwest of Yen-chuang, Shan-tung. U. S. Nat. Mus., Cat. No. 525060. 

 i b, ic, and id. Summit, back, and side views of a small ventral valve, X 15. From same locality as 



fii. i. U. S. Nat. Mus., Cat. No. 525066. 

 le. Partly exfoliated dorsal valve, X 15. From Locality 02, Middle Cambrian, Kiu-lung group, 



south of Yen-chuang, Shan-tung. U. S. Nat. Mus., Cat. No. 52508. 

 i/. Summit view of a ventral valve crushed down in a fine arenaceous shale, X 8. Locality 35 p. Middle 



Cambrian, Fu-chou series, Liau-tung, Manchuria. U. S. Nat. Mus., Cat. No. 57764. 

 Acrotreta shantungensis Walcott (p. 76). 



2. 20, and 26. Summit, side, and back views of ventral valve, X 15. U.S. Nat. Mus., Cat. No. 525120. 

 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. 525126. 

 id. Exterior of a dorsal valve, X 15. U. S. Nat. Mus., Cat. No. 525126. 

 ze. Interior of a dorsal valve, X 15. U. S. Nat. Mus., Cat. No. 525120*. 



The specimens illustrated are from Locality C 62, 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 C 56, Upper Cambrian, 



Ch'au-mi-tien limestone, west of Tsi-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 C 61, Upper Cambrian, Kiu-lung 



group, southwest of Yen-chuang, Shan-tung. U. S. Nat. Mus., Cat. No. 525580. 

 40. 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. 525570. 

 46. Partly exfoliated dorsal valve, X 3. U. S. Nat. Mus., Cat. No. 525576. 

 4C. Enlargement of the radiating costse toward the front margin of the shell. U. S. Nat. Mus., Cat. 



No. 525586. 

 Figures ^a-c are of specimens from Locality 036, Upper Cambrian, Ch'au-mi-tien limestone, at 



Ch'au-mi-tien, Shan-tung. 

 Billingsella richthofeni Walcott (p. So). 



5. Summit view and side outline of a ventral valve, X 4. From Locality C 3, 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. 525450. 



6a. Enlargement of radiating coste near front margin, X 3- U. S. Nat. Mus., Cat. No. 525456. 

 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. 525510. 



70. 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. 525516. 



From Locality C 64, 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. 525460. 

 80. Small, partly exfoliated dorsal valve, X 3. U. S. Nat. Mus., Cat. No. 525466. 



86. Cast of interior of a dorsal valve, showing cast of strong median ridge and vascular sinuses, X 2. 



U. S. Nat. Mus., Cat. No. 52546*:. 

 The three specimens illustrated are from Locality 064, Upper Cambrian, Kiu-lung group, southwest 



of Yen-chuang, Shan-tung. 

 Eoorthis sp. undt. (6) (Walcott) (p. 85). 



9. Exterior of a partly abraded ventral valve, X 3. From Locality 071, Middle Cambrian, Ki-chou 



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 075, Middle Cambrian, Ki-ch6u 



formation, south of Wu-t'ai-hien, 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 C 64, Upper Cambrian, Kiu- 



lung group, southwest of Yen-chuang, Shan-tung. U. S. Nat. Mus., Cat. No. 52549. 

 1 10. Copy of portion of original figure from Kayser [1883, pi. in, fig. i]. 

 Eoorthis pagoda (Walcott) (p. 83). 



12 and 1 20. Exterior views of partly exfoliated ventral valves, X 2. U. S. Nat. Mus., Cat. Nos. 



525520 and 525526. 



126. Partly exfoliated dorsal valve, X 2. U. S. Nat. Mus., Cat. No. 525526. 

 I2C. Interior of a dorsal valve that has been slightly injured by abrasion, X 4. U. S. Nat. Mus., Cat. 



No. 525520". 



120". Enlargement of the cast of a portion of the outer surface, X 6. U. S. Nat. Mus., Cat. No. 52552?. 

 All of the specimens illustrated are from Locality 056, Upper Cambrian, Ch'au-mi-tien 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 026, 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. 525640. 

 140. Partly exfoliated dorsal valve, with side outline, X 2.5. U. S. Nat. Mus., Cat. No. 525646. 

 146. Smooth form of a dorsal valve, preserving a portion of the outer surface of the shell, X 3. U. S. 



Nat. Mus., Cat. No. 52564*:. 

 From Locality 064, Upper Cambrian, Kiu-lung group, southwest of Yen-chuang, Shan-tung. 



236 



