PLATE XV. 



PTILOCELLA. 



Ptilocella parallela, Hall. 

 Fig. 1. A specimen, natural size. 

 Fig. 2. A portion of the preceding . x6 . 

 Fig. 3. A transverse section of a frond. x6. 



Hamilton group. Ontario county^ JV. Y. 



PTILODICTYA. 



(Page 541.) 

 Ptilodicfta plumea, Hall. 

 Fig. 4. A specimen, natural size. 



Fig. 5. An enlargement of a portion of the surface. x6. 

 Fig. 6. An enlargement of the base of Fig. 4. x6. • 

 Hamilton group. West Hamhurgh^ N, Y, 



Ptilodictya retipobmis, Hall. 



Fig. 1, An enlargement of the surface, showing the form and dispo- 

 sition of the cell apertures and the vestibular areas . x6. 

 Hamilton group. Alden, N. Y, 



PH^NOPORA. 



(Page 541.) 



Ph^nopoea libata, Hall. 



Fig. 8 . A portion of a specimen. x6. 



Lower Helderberg group. Clarksville, JV. Y. 



Phjenopora tenuis. Hall. 



Fig. 9. A specimen, natural size. 



Fig. 10. An enlargement of the preceding. x6. 



Lower Helderberg group . Albany county j JV, Y, 



ACROGENIA.. 



(Page 544.) 



AcROGENIA PROLIFERA, Hall. 



Figs. 11, 12. Segments, natural size . 



Fig. 13. A perfect specimen, natural size. 



Fig. 14. An enlargement of the roots of the preceding specimen. x6. 



648 



