3 8o 



THE 0RD0V1CIAN PERIOD 



of Ordovician genera and species. Like other Palaeozoic Corals, 

 these are characteristically different from the reef-builders of the 



present day in showing a marked 

 bilateral symmetry and having the 

 septa arranged in multiples of four 

 ( Tetracoralla) . Large, solitary cup- 

 corals, like Streptelasma, and com- 

 pound colonies, like Favistella and 

 Co/umnaria, are examples of the 

 range of differences among these 

 early corals. 



The Echinodermata have greatly 

 increased in importance, and except 

 the Echinoids, all the main subdivi- 

 sions of the group are represented. 

 The Cystidea, which we have already 

 found in the Cambrian, attain their 

 greatest development in the Ordo- 

 vician. In these curious animals the 

 body is either irregularly shaped, or 

 symmetrical, with a short, tapering 

 stem, by which the animal was at- 

 tached to the sea-floor, and weakly developed arms. The body, 

 or calyx, is made up of a number of calcareous plates ; when 

 these plates are very numerous, they are of irregular size and 

 arrangement (PI. II, Fig. 6), while the forms with few plates have 

 them of a definite number, size, and shape (PI. Ill, Fig. i). 

 Some of the more regular Cystidea have much resemblance to 

 the true Crinoids. The latter make their first appearance in the 

 upper part of the Cambrian, but in the Ordovician they greatly 

 increase in numbers and importance, though less abundant than 

 they afterwards became. These animals (PL II, Fig. 5) have a 

 symmetrical calyx, with long, branching arms ; the number and 

 arrangement of the component plates are definite and character- 

 istic for each genus. Most, but not all, of the Crinoids have a 

 long, jointed stem, by which they were attached to the sea-bottom. 



Fig. 137. — Ordovician Coral, 

 Favistella stcllata. Hudson Stage, 

 New York. (Hall.) 



