PLATE XI. 

 INTRAPORA. 



(Page 535.) 

 Intbapoea pittkolata, Hall. 



Figs. 1-5. Fragments of fronds, natural size, showing variations in 

 form. 



Fig. 6. A portion of a frond. x6. 



Fig. Y. A still further enlargement from the preceding figure. xl8. 



Fig. 8. An enlargement from another specimen having circular aper- 

 tures and fewer interapertural pits. xlS. 



Fig. 9. A transverse specimen of one-half of a specimen. x6. 

 Upper Helderberg group. Falls of the Ohio, 



SEMIOPORA. 



(Page 535.) 



SeMIOPORA BISTIGMATA, Hall. 



Fig. 10. A frond, natural size. 

 Fig. 11. A portion of the preceding. x6. 

 Hamilton group. New York. 



TJENIOPORA. 



(Page 533.) 

 Tjsniopora exigua, Nicholson. 

 Fig. 12. A portion of the concave face. x6. 

 Fig. 13. A portion of the angular face. x6. 

 Fig. 14. The mesotheca worn away, showing the under portion of 



the cell tubes. x6. 

 Fig. 15. A transverse section of a branch. x6. 



Fig. 16. A side view of the cell tubes, showing their form and mode 

 of growth. 

 Hamilton group. Various localities in New York. 



640 



