PLATE XXI. 
Fenestella Thyene. 
Page 50. . 
Fig-. 1. A fragment of the non-celluliferous face of a frond, natural size. 
Fig. 2. An enlargement from the preceding. 
Fig. 3. An enlargement of the non-celluliferous face showing more slender branches. 
Lower Helderberg group. (Schoharie, IS. Y. 
Fig. 4. An enlargement of the celluliferous face with an outline section of a branch at the left hand of the 
figure. 
Fig. 5. A large funnel-shaped frond, natural size. 
Lower Helderberg group. Schoharie, N. Y. 
Fenestella Idalia. 
Page 52. 
Fig. 6. The non-celluliferous face of a funnel-shaped frond, natural size. 
Fig. 7. An enlargement of the non-celluliferous face. 
Lower Helderberg group. Clarksville, JS. Y. 
Fig. 8. An enlargement of the non-celluliferous face showing slender striate branches. 
Fig. 9. An enlargement of the non-celluliferous face of a frond, with stouter branches. 
Fenestlla Coronis. 
Page 51. 
Fig. 10. A fragment of the non-celluliferous face of a frond, natural size. 
Fig. 11. An enlargement of the non-celluliferous face showing the thickened base, and the entire surface free 
from strise. 
Fig. 12. An enlargement from the non-celluliferous face. The upper part of the figure, from the outer ex¬ 
tension of the frond, shows elongate fenestrules and strongly striate granulose branches. 
Fig. 13. An enlargement from the celluliferous face of the specimen fig. 11. 
Lower Helderberg group. Near Clarksville, N. Y. 
Fenestella (Unitrypa) precursor. 
Page 54-. 
Fig. 14. An enlargement bf the non-celluliferous face of a frond. 
Fig. 15. An enlargement of the celluliferous face, showing the carinate branches, with the carinse expanded 
above and covering the cell apertures, except in the upper part of the figure where they have 
been removed. 
Fig. 16. An enlargement of the celluliferous face, with the carinse removed, exposing the cell apertures. 
Fig. 17. An enlargement showing the alate carinse, which in some parts are united by the extension of the 
marginal nodes into slender connecting bars. 
Fig. 18. An enlargement from the ends of the branches showing- the form of the carinse with their lateral 
expansions. 
Lower Helderberg"group. Near Clarksville, N. Y. 
Fenestella quadrula. 
Page 53. 
Fig. 19. A fragment of the non-celluliferous face of a frond, natural size. 
Fig. 20. An enlargement from the preceding. 
Fig. 21. An enlargement of the celluliferous face with the carinse broken oft', leaving the branches appar¬ 
ently grooved. 
Fig. 22. An enlargement of the celluliferous face, showing the presence of the alate carinse with the mar¬ 
ginal crenulations. 
Lower Helderberg- group. Near Clarksville, N. Y. 
