PLATE XVIII. 
Fenestella (Polypora) compacta. 
Page 63. 
See Plate 22. 
Fig-. 1. A fragment from near the base of a frond, natural size. 
Fig. 2. Aii enlargement of the surface of the non-celluliferous side. 
Fig-. 8. An enlargement of a portion of the celluliferous face of the frond. 
Fenestella (Polypora) Arta. 
Page 63. 
Fig 4. A fragment of a frond, showing a difference in the size of the branches and of the fenestrules on 
the non-celluliferous side, natural size. 
Fig-. 5. An enlargement of the non-celluliferous face, with slender branches and large fenestrules. 
Fig. 6. The non-celluliferous face of a fragment, natural size. 
Fig. 7. An enlargement from the preceding, showing stouter branches and smaller fenestrules than fig. 5. 
Fenestella (Polypora) obliqua. 
Page 65. 
Fig. 8. A fragment of the celluliferous face, natural size. 
Fig-. 9. An enlargement from the above, showing cell apertures, etc. 
Lower Helderberg group. Near Catskill, N. Y. 
Fenestella (Polypora) paxillata. 
Page 65. 
Fig. 10. A fragment showing the non-celluliferous face of the frond. 
Fig. 11. An enlargement from the above ; one branch is decorticated showing the interior structure. 
Fig. 12. An enlargement of the celluliferous side of a specimen. 
Lower Helderberg group. Clarksville, N. Y. 
Fenestella (Polypora) sp. indet. 
Fig. 18. An enlargement from a fragment worn down from the non-celluliferous face and showing two, three 
and four rang-es of cells. 
Fenestella (Polypora) com press a. 
Page 61. 
Fig. 14. A fragment, natural size, presenting the non-celluliferous face. 
Lower Helderberg g-roup. Clarksville, N. Y. 
Fig. 15. An enlargement from the preceding. 
Fi<>-. 16. An enlargement of the non-celluliferous side of a specimen, showing the finely granulose striee. 
Fig. 17. An enlargement from the celluliferous face of a frond, showing the obliquely upward opening of 
the pores. The surface is modified from weathering. 
Lower Helderberg group. Clarksville, N. Y. 
Fig. 18. An enlargement of the celluliferous face of a fragment, showing the cell apertures opening rect¬ 
angularly to the axis. 
Lower Helderberg group. Catskill creek, N. Y. 
Fenestella (Polypora) Lil^ea. 
Page 62. 
Fig. 19. A fragment of the celluliferous face of a frond, natural size. 
Fig. 20. An enlargement from the preceding. 
Lower Helderberg group. Clarksville, N. Y. 
Fig. 21. A fragment of a frond, natural size, showing the celluliferous face. 
Fig. 22. An enlargement of the preceding. 
Lower Helderberg group. Schoharie, N. Y. 
