THE ORDOVICIAN PERIOD 93 



Hexacoralla or Octacoralla. Modern Corals are nearly all pro- 

 fusely branched and the polyps are very small, while Paleozoic 

 Corals were rarely branched and the polyps were much larger, the 

 cup Corals usually ranging from half an inch to a foot or more in 

 length. All three types of Corals above mentioned existed in the 

 Ordovician, but solitary cup Corals were predominant. Com- 

 pound forms, especially honeycomb Corals, were sometimes locally 

 abundant. Among modern Corals the compound or colonizing 

 forms are by far more common than the solitary forms. 



Echinoderms. — All the classes of the Echinoderms were re- 

 presented in the Ordovician, and all of these but the Cystoids and 



b c 



Fig. 50 

 Ordovician Echinoderms: a, Cystoid, Pleurocystis filitextus; 

 b, Crinoid, Glyptocrinus dyeri; c, Asterozoan, Paleasterina 

 stellata. (a, c, after Billings; b, after Meek.) 



Holothuroids made their first appearance. Cystoids (Fig. 50a) 

 reached their climax of development in this period, though they 

 did not become extinct till the Devonian. Blastoids were rare 

 and represented by very primitive forms, with distinct Cystoid 

 affinities. In fact the Blastoids assumed little importance till the 

 Mississippian. Crinoids (Fig. 50b) became prominent, and, be- 

 cause of their hard parts, were well suited for fossilization, though, 

 on account of their highly segmented character, they usually fell 

 apart after the decay of the soft parts, and consequently entire 

 specimens are not common. Ophiuroids, Asteroids (Fig. 50c), 

 and Echinoids were uncommon, the latter being represented by 

 very primitive forms. Holothuroids are not known as fossils. 



