532 GEOLOGY 



semble coral that they have often been mistaken for it. The bry- 

 ozoans became abundant in the middle and later portions of the 

 period, when their secretions contributed much to the limestone. 



The Echinoderms . One division of the echinoderms, the cystoids, 

 reached its climax before the close of the Ordovician period; another, 

 the crinoids, became prominent, and others (asteroids, ophiurians, 

 and echinoids, p. 945) had made their appearance. The cystoids, 

 (a, b, and c, Fig. 392) with their irregular forms, were the most 

 primitive, and gave place in time to the more symmetrical crinoids 

 (Fig. 392, d to k), which may be likened to star-fishes turned face 

 uppermost and fixed to the sea-bottom by a calcareous stem 

 attached to the center of the back. The crinoids so closely resem- 

 bled a flower in form, that the familiar name " sea-lily" is not 

 inappropriate. The crinoids were excellent subjects for fossiliza- 

 tion, save that, after the tissues decayed, the constituent hard 

 portions fell apart easily so that perfect specimens are rare. Some 

 limestone is made up largely of crinoidal fragments. 



The structure of the cystoids (Fig. 392, a to c) was similar to 

 that of the crinoids, but the body was unsymmetrical both in form 

 and in the arrangement of the plates. Little can be said of their 

 evolution, for their forms are so heterogeneous and their functions 

 so little known that it is not clear what constituted progress. The 

 other echinoderms attained their principal development later. 



The ccelenterates. Corals are few in the lower part of the sys- 

 tem, and though more abundant in higher beds, are nowhere a lead- 

 ing element in the fauna. Most of them belonged to the simpler 

 horn-shaped type (Fig. 393, a), but compound and colonial corals 

 were present. The most important development of the coelenterates 

 was the rise of the graptolites (Fig. 394), whose important function 

 in correlation has been referred to. 



Other forms. Sponges were present and sometimes attained 

 notable size (Fig. 395). The record of annelids is more meager than 

 in the Cambrian, perhaps because the calcareous sea-bottom of the 

 Ordovician was less congenial to them than the Cambrian sands. 

 They are represented by burrows and by teeth (Fig. 396). Proto- 

 zoans were probably present, but their minute and fragile shells 

 can be recognized only with some uncertainty. 



