SILURIAN ANIMALS. 



309 



called graptolites. Sometimes the cells are arranged on one side of the 

 axis, sometimes on both sides, sometimes the axis is divided. What- 

 ever be their affinities, they are of great importance, inasmuch as they 

 are entirely characteristic of the Silurian age, and those tvith cells on 

 both sides, of the Lower Silurian and Cambrian. The twin graptolites 

 (Fig. 315) are also wholly characteristic of Lower Silurian. 



Fig. 318. 



Fig. 319. 



Fi.,8. 318, 319. 



-Graptolites: 318. Dendrograptus Hallianus (after Hall). 319. Graptolites Clinto- 

 nensis (after Hall). 



Polyzoa. — There are many kinds of compound coralline animals, 

 probably allied to the Bryozoa (sea-mats) (Fig. 320) of our present seas, 

 found in the Silurian. The doubtful 

 affinities of these Palaeozoic forms, and 

 the difficulty of separating them sharply 

 from certain forms of true corals on the 

 one hand, and from certain forms of 

 graptolites on the other, seem to require 

 their notice in this connection, although 

 their affinities are probably molluscoid. 

 Two of the Silurian forms are represented 

 in Figs. 321 and 322. 



Echinoderms. — During Silurian times 

 the class of Echinoderms was represented 

 principally by Crinoids. A Crinoid is 

 a stemmed Echinoderm, usually with 

 branching arms. The animal consists of a long jointed stalk, rooted to 

 the sea-bottom, and bearing atop a rounded or pear-shaped body, cov- 

 ered with calcareous plates (calyx), from the margin of which spring the 

 arms, which may be long and profusely branched, or short and simple, 

 or absent altogether. In the middle of the calyx, between the bases of 

 the arms, is placed the mouth. Their general structure and appear- 

 ance will be better understood by examination of the following figures 

 (323-325) of living Crinoids. 



Fig. 320.— Living Polyzoa: Flustratmn- 

 cata: a, natural size; b, enlarged to 

 show the cells. 



