PLATE 52. 
1. 612. 25. Orthis pisum. (Pag- 250.) 
1 a, b. Young specimens of this species. 
1 c. The ventral valve of a full grown individual, showing the beak of the dorsal valve. 
1 d. Profile view of the same, the shell slightly compressed. 1 e. View of the beaks of the same. 
2. 613. 26. Orthis pyramidalis. (Pag. 251.) 
2 a, b, c, d. Dorsal, ventral, cardinal and profile views of a specimen of the ordinary size. 
2 e, f, g, h. Similar views of a much larger shell. 
2 i. An enlargement of the surface, showing the sharp plications crossed by concentric striae. 
3. 440. 22. Orthis elegantula. (Pag. 252.) 
3 a, b. The dorsal valves of young individuals. 
3 c. The dorsal valve of a full grown individual, having the beak somewhat more extended than in other 
specimens. 
3 d. Yentral view of the same specimen. 3 e. Profile view of the same specimen. 
3 /. Cardinal view of the same, showing the elevation of the dorsal valve. 
3 g. Yentral view of a shorter specimen. 3 h. Cardinal view of the same. 
3 i. Cast of a dorsal valve of an elongated specimen. 3 fc. Cardinal view of the same. 
3 l, m. The interior of the dorsal valves of a young and full grown individual. 
3 n. The interior of a ventral valve, showing the muscular impresssion, lamellae, etc. 
3 o. The same enlarged. 3 p. The interior of a similar shell, showing a slight variation in character. 
3 r. Cardinal view of the same, showing the dental lamellae, and the callosity at the apex of the foramen. 
3 s. An enlargement of the surface. 
4. 614. 27. Orthis hybrida. (Pag. 253.) 
4 a, b, c. Figures of specimens showing three gradations of size. 
4 d, e. Cardinal and profile views of fig. 4 c. 
4 /. A specimen having the valves more convex than usual. 
4 g. Profile view of the same specimen, showing the nearly equal convexity of the valves. 
4 h. A cardinal view of a specimen, showing the convexity of the dorsal valve'. 
4 i , k. The interior of the dorsal valve of this species, showing the muscular impression divided by a longi¬ 
tudinal ridge. 
4 l. An enlargement of the muscular impression of the preceding specimen. 
4 m. The interior of the ventral valve, with a scarcely defined muscular impression. 
4 n . A cast of the ventral valve of this species, showing the bilobate character of the muscular impression, 
the cavities of the teeth, etc. 
4 o. An enlargement of the surface, showing the longitudinal diverging and fine concentric striae. 
5. 615. 28. Orthis puncto-striata. (Pag. 254.) 
5 a. Yiew of the ventral valve of a small specimen, showing the beak of the dorsal valve, and a part of the 
area above it. 5 b. A profile view of the same specimen. 
5 c. A cardinal view of a larger specimen-, showing the great convexity of the valves. 
5 d. A large dorsal valve which has been flattened by pressure. 5 e. The interior of the same. 
5 /. Several of the striae enlarged, showing the puncta between. 
6. 616. 29. Orthis flabellulum, Var.? (Pag. 254.) 
6 a, b, c. Dorsal, profile and ventral views of a compressed specimen. 
6 d. The interior of a ventral valve. 6 e. A cast of a ventral valve. 
6 /. Cardinal view of a specimen, which is partially a cast. 
6 g. Cardinal view of the specimen fig. 6 a. 
7. 616. 29. Orthis flabellulum? Var.? (Pag. 255.) 
7 a, b. Dorsal and ventral views of the specimen. 
7 c. Profile view of the same. 7 d. Cardinal view the same. 
8. 617. 30. Orthis fabciata. (Pag. 255.) 
8 a. A cast of the ventral valve, showing an extended hinge-line. 8 b. A cast of the dorsal valve. 
