PLATE XVII. 

 Ptilodiotya lirata. 



Page 38. 

 See Plate 23 A. 

 Fig. 1. The base of an individual, natural size. 



Lower HeWerberg group. ClarksviUe, N. Y. 

 Fig. 2. An enlargement from the preceding. 

 Fig. 3. A fi-agment of a fi-ond, natui-al size. 



Lower Helderberg group. ClarksvilU, N. ¥. 

 Pig. 4. An enlargement from the preceding. 



RhINODICTYA ? (JRANUL08A. 

 Page 40. 

 See Plate 23 a. 

 Pig. S. A slender stipe, natural size. 

 Pig. 6. -An enlargement of a poHion of a stipe. 



Lower Helderberg group. ClarkmnUe, N. T. 



Ptilodictya tenuis. 



Page 39. 

 See Plates 13, 23 a. 



Pig. 7. A narrow curving stipe, natural size. 



Lower Helderberg group. ClarksviUe, N. T. 

 Fig. 8. An enlargement from the preceding. 

 Fig. 9. A base of a specimen, natural size. 



Lower Helderberg group. ClarksvUle, N. T. 

 Fig. 10. An enlargement of the pi-eceding, showing the minute intercellular pits. 



Fig. 11. An enlargement from another specimen, differing somewhat in the size and shape of the cell 

 apertures from figs. 8 and 10. 



Lower Helderberg group. ClarksviUe, iV. Y. 

 Fig. 12. A longitudinal section, showing origin and mode of growth of the cellules. 



Lower Helderberg group. ClarksviUe, N. Y. 



Ptilodiciya nebulosa. 



Page 40. 

 Fig. 13. An enlargement of a portion of a fragment. 



Lower Helderberg group. ClarksviUe, N. Y. 

 Fig. 14. The lower portion of a frond with the base of attachment. 



Lower Helderberg group. Near Catskill, N. Y. 

 Fig. 15. An enlargement from the preceding, showing the form and size of cell apertures, and nodes with 



partially ban-en. spaces. 

 Fig. 16. An enlargement fi-om another specimen, showing larger cell apertures which are often polygonal ; 

 the elevated nodes are larger and are covered with cellules as on the other parts of the frond. 

 Lower Helderberg group. Near Catskill, N. Y. 

 Fig. 17. An irregular fragment, natural size. 



Lower Helderberg group, fichoharie, N. Y. 

 Fig. 18. An enlargement, showing polygonal cell apertures and strong nodes on the surface. 



