504 



HISTORICAL GEOLOGY. 



2. Trenton Period. 



1. Trenton Epoch. 



Flakts. — Two of the forms referred to Algae or Seaweeds are here repre- 

 sented. Tliey are much like the so-called Fucoids of earlier and later time ; 

 but whether of vegetable origin is questioned. 



Alg^. — Fig. 6i0, Buthotrephis gracilis; 641, B. succulens. Hall. 



Specimens of supposed terrestrial plants have been reported from the' 

 Cincinnati beds of Ohio and Kentucky ; but no certain evidence from fossils- 

 of vegetation over the land had been found up to the year 1894. 



642. 



643. 



Sponge. — Brachiospongia digitata. Beecher. 



Animals. 1. Spongiozoans. — A large branching Hexactinellid Sponge, 

 Brachiospongia digitata, from the Trenton of Kentucky and Tennessee, is 

 represented in different positions in Figs. 642, 643. The number of short 

 branchings varies from 8 to 12, and specimens having 12 are sometimes 

 10|- inches in diameter. 



2. Hydrozoans are represented by Graptolites (647, a) and Stromatoporids. 



3. Actinozoans comprise Cyathophylloid Corals like Fig. 644, Streptelasma 

 cornicidum H. ; and tabulate Corals as Columnaria alveolata H., — the term 

 tabulate referring to the horizontal partitions seen in vertical sections of the 

 columnar cells (Fig. 645). 



