THE ORDOVICIAN PERIOD 



531 



Fig. 393. Ordovician Corals: a, Streptelasma corniculum Hall; b, Colum- 

 naria alveolata Goldf. Both simple and compound corals were present 

 in the Ordovician faunas, but they rarely formed great reefs as in later 

 periods. 



Fig. 394. Ordovician Graptolites: a, Dichograptus octobrachiatus (Hall); 

 b, Reteograptus eucharis Hall; c, Phyllograptus ilicifolius Hall; d, Dip- 

 lograptus pristis (Hall) (restored by Ruedemann); e, Tetragraptus 

 fruticosus (Hall); /, Climacograptus bicornis (Hall); g, Didymograptus 

 nitidus (Hall); h, 1'etragraptus bigsbyi (Hall); i, Phyllograptus typus Hall; 

 y, Holograptus richardsoni (Hall). 



