548 GEOLOGY 



most forms of shallow-water life resulted in an expansional evolu- 

 tion which produced the Niagara fauna. The families and classes 

 were much the same as in the Ordovician period, but most of the 

 genera were new, and nearly all the species. In general there was 

 a biological advance, though this was not true of all classes. Only 

 the more conspicuous, features of the changes will be noted. 



The echinoderms. A distinguishing feature of the Silurian fauna 

 was the rich and varied development of the echinoderms, involving 

 at once the rise or the decline of previous forms, and the introduction 

 of new ones. The great feature of the period, in connection with 

 the echinoderms, was the rise of the crinoids. They attained such 

 abundance in certain congenial localities that their fragments 

 formed the larger part of the limestone. These spots were veritable 

 " flower-beds" of " stone lilies," where beautiful and varied forms 

 grew in groves, as it were. The assemblage of species at each of 

 these localities had its own peculiarities, but the genera were the 

 same or similar. A few Silurian crinoids are shown in Fig. 400, 

 but no limited number of figures can do justice to their variety and 

 beauty. Notwithstanding various signs of progress, many of the 

 more primitive characters remained, indicating that the class had 

 not yet reached its climax. 



Cystoids were still abundant, and the true blastoids now appear 

 for the first time. Starfishes appear to have made little progress 

 and to have had no large place in the fauna, and the serpent-stars 

 and the echinoids had even less. The slow development of these 

 types which were prominent much later, possibly represents a 

 general fact, viz., that great classes were really slow in their evolu- 

 tion, however suddenly they may seem to have come into existence. 

 Perhaps the ascent of the cystoids and crinoids to their climaxes 

 would be found to be as slow as those of their kin, if we could trace 

 their history back to its beginning. A little greater imperfection 

 in the fossil record would have eliminated all trace of the serpent- 

 stars and sea-urchins in these early periods, and would have made 

 their appearance in abundance at a later period seem sudden and 

 remarkable. 



The brachiopods. The brachiopods stood the vicissitudes of the 

 passage from the Ordovician to the Silurian with no loss of prestige, 



