PLATE 20. 
Fig. 1. 434. 13. Lingula oblonga. (Pag. 54.) 
1 a. A young individual of this species. 1 ft; A specimen of the ordinary size of this shell. 
1 c. Specimen longer than the usual form. 
1 d. A portion of the surface enlarged, showing the longitudinal striae and concentric lines of growth. 
Fig. 2. 435. 14. Lingula oblata. (Pag. 54.) 
2 a. A figure of an individual of the ordinary size of this species. 
2 ft. A portion of the surface enlarged. 
2 c. The inner surface of a specimen, showing the longitudinal depressed line. 
Fig. 3. 436. 15. Lingula perovata. (Pag. 55.) 
.3 a. Specimen natural size. 3 ft. The specimen enlarged. 
Fig. 4. 437. 16. Lingula lamellata. (Pag. 55.) 
4 a. A single valve, preserving the form and character of the shell very perfectly. 
4 ft. A portion of the surface enlarged. 
Fig. 5. 438. 17. Lingula acutirostra. ( Pag. 56.) 
F'ig., 6. 439. 21. Okthis circulus. (Pag. 56.) 
6 a. A view of the ventral valve,, showing the beak of the dorsal valve. 
6 6. View of the dorsal valve. 6 c. Profile view of the shell. 6 d. Front view of the same. 
6 e. Cardinal view of the same. 6 /. Cast of the dorsal valve of this species. 
6 g. The striae enlarged, showing their mode of bifurcation and increase. 
Fig. 7. 440. 22. Orthis elegantula? Var. (Pag.57.) 
7 a. Dorsal valve of a specimen of medium size, somewhat distorted by pressure. 
7 ft. Hinge-line and beak of dorsal valve. 7 c. Dorsal valve of a large individual. 
7 e. The cast of a dorsal valve of a small individual. 
7 f. Cast of the dorsal valve somewhat compressed. 
7 g. Dorsal valve with the shell partially removed. 7 ft. Dorsal area of 7 g. 
7 i. Showing the flattened stria;. 7 k. Several of the striae enlarged. 
Fig. 8. 441. 23. Orthis trinucleus. (Pag. 58.) 
8 a. Interior of ventral valve. 8 ft. Cast of ventral valve. . 
Fig. 9. 442. 24. Orthis tenuidens. ( Pag. 58.) 
9 a. Ventral valve with the shell partially removed. 
9 6. A more perfect cast, showing the form of the muscular impression. 
9 c. The last enlarged. 9 d. The impression of a dorsal valve. 9 e. The last enlarged. 
Fig. 10. 469. 40. Atryfa gibbosa. ( Pag. 79.) 
10 a c. Casts of the dorsal valve. 10 ft. Cast of the ventral valve. 
10 d. Cast of the dorsal valve, differing slightly from the preceding. 
10 e. Profile view of the dofsal valve. 
