PLATE 58 a. 
Page 
Pigs. 1-8. GrYPIDTJLA OCCIDENTALIS. . 880 
1-5. Dorsal, ventral, profile, front and cardinal views of a very characteristic specimen, show¬ 
ing plications extending some distance from the margin of the shell. 
G, 7. Profile and front views of another individual which is entirely destitute of plications. 
8. An enlargement of the upper half of the shell, showing the area and foramen. 
Pigs. 9-20. Rensselaeria (?) johanni. 385 
9. A dorsal valve of a young and moderately convex specimen. 
10. 11. Dorsal and profile views of a larger and very ventricose specimen. 
12. Ventral view of a specimen of medium size. 
13-15. Dorsal, ventral and profile views of a crushed and somewhat distorted specimen. 
16, 17. Dorsal and ventral views of a characteristic specimen, somewhat larger than the 
ordinary size. 
18. Ventral valve of a very broad and unusual form. 
19. Dorsal view of a cast, showing muscular impressions, dental cavities, etc. 
20. Ventral view of a cast, showing the muscular and hinge-plate impressions. 
Figs. 21-24. Amphigenia elongata... 383 
21 - 24. Views of specimens, showing gradations in size and varieties of form in young indi¬ 
viduals of the species. 
Pigs. 25-^27. Amphigenia elongata var. undulata. 384 
25. Ventral view of a specimen from Mackinac, Michigan. 
26. Interior of an imperfect ventral valve from the drift of Michigan, from Dr. C. Rominger. 
27. Cast of a large ventral valve from the same source. 
