PLATE 53 a. 
Page 
Figs. 1 -14, 18, 24, 25. Atrypa spinosa. . 822 
1. Dorsal view of a young individual, from the Corniferous limestone, showing the perforated 
beak, and the extended lamellae near the margin of the shell. 
2-4. Dorsal, ventral and profile views of a specimen, of the ordinary form and size of those 
from the Upper Helderberg limestones of New York. 
5. Dorsal view of a large specimen from the Upper Helderberg limestone of New York. 
6. The interior of a ventral valve, from the Falls of the Ohio. 
7. The interior of an imperfect dorsal valve, from the Falls of the Ohio, showing the socket 
plates and muscular impressions; enlarged. 
8. 9. Views of a small and medium sized specimen, from the Hamilton shales, showing the 
spines. 
10, 12. Dorsal, ventral and profile views of a specimen of ordinary size, showing the surface 
denuded of the spines; from the Hamilton shales. 
13. Dorsal view of a large specimen without spines, from the Hamilton group. 
14. The interior of a ventral valve, showing muscular impressions, teeth, etc. 
18. Dox*sal view of a specimen, from the Chemung group, of New York. 
24. Dorsal side of a specimen, showing the spires, crura and loop. 
25. Ventral side of the same specimen showing the position of the spires, loop and attach¬ 
ment of the crura to the hinge-plate. From the collection of Mr. R. P. Whitfield. 
Figs. 15-17. Atrypa hystrix. 826 
15. Ventral view of a small specimen which preserves the marginal spines. 
16. View of an individual preserving several spines to a length of nearly three-fourths of an inch. 
17. View of an individual having very strong symmetrical plications. 
Figs. 19-21. Atrypa aspera var. occidentalis. 
19-21. Ventral, profile and dorsal view of a specimen, showing the prevailing form of this spe¬ 
cies as it occurs at Independence, Iowa. 
Figs. 22, 23* Atrypa reticularis. 316 
22. The vential side of a specimen, showing the position of the spiral cones, with their attach¬ 
ment to the hinge-plate, and the loop connecting the spires. 
23. Dorsal side of the same specimen. From the collection of Mr. R. P. Whitfield. 
