PLATE 3. 
Figs. 1, 2. Pholidops . oyalis . 
1, 2. Vertical and profile views of a specimen retaining both valves. From the Niagara group, 
ol’ Waldron, Ind. Enlarged to five diameters. 
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Fig. 3. Pholidops arenariA. (See note on 32) 
A cast of the valve from the Oriskany sandstone, enlarged to three diameters. See p. 413. 
Figs. 4, 5. Pholidops areolata... 31 
4. An impression of the dorsal valve. 
5. An impression of the ventral valve. The figures enlarged to three diameters. 
Figs. 6-9. Pholidops hamiltoni.®. 32 
6. The exterior of the shell, enlarged to four diameters. 
7. The interior of ventral valve, showing the muscular impressions. 
8. An impression of the ventral valve. 
9. The opposite or dorsal valve of the same species, enlarged to eight diameters. 
Fig. 10. Pholidops oblata. 413 
10. An impression (apparently of the dorsal valve), showing the muscular impression; enlarged 
to four diameters. 
Fig. 11. Pholidops ? lamellosa. .. 413 
11. Ventral ? valve of the specimen; enlarged to two diameters. 
Fig. 12. Crania aurora. 27 
12. An impression of a ventral valve. 
Figs. 13-16. Crania crenistriata. 28 
13. The dorsal valve of a small individual. 
14. The dorsal valve of a large individual, the margins having been crushed. 
15. Vertical view of a broadly elliptical dorsal valve. 
16. Profile of the same. 
Figs: 17-23. Crania hamiltoni^e... 27 
17. Vertical view of a dorsal valve. 
18. Profile of the above. 
19. Two individuals attached to the surface'of a shell of Tropidoleptus, one of them having 
carried the form of the ribs of the shell into its growth. 
20. The interior of a ventral valve. 
21. A group of ventral valves attached to the surface of Microdon bellastriata. 
22. A ventral valve having the cardinal border modified in its outline by the presence of Spi- 
rorbis angulata a, a, attached to the same shell surface with the Crania. 
23. An enlargement of the specimen fig. 22,Intended to show the muscular and vascular markings 
more distinctly. 
