i6 



NORTH AMERICAN INDEX FOSSILS. 



spicular threads and rarely ornamented with inconspicuous nodes 

 Found as internal molds. 



Characteristic of the Chemung and occurs also in the upper 

 Portage and the Keokuk. 



5. P. prismatica Hall. (Fig. 24.) Upper Devonic 



Often without perceptible expansion for a considerable distance 

 but usually expanding to an aperture whose diameter is the greatest 



Fig. 24. Prismodictya prismatica (after 

 Hall). 



Fig. 25. Hydnoceras tuberosum (after 

 Hall). 



width of the sponge. Sometimes curved or twisted. Prism faces 

 flat and each marked with one or two clearly defined spicular 

 threads and other minor ones. 



Chemung of New York, Pennsylvania. 



