THE SILURIAN PERIOD. 401 



added strength to it. Spines were occasional features and doubtless 

 served a defensive purpose. Some species were notably ornamented 

 by various figures upon the exterior of the plates, the function of which 

 can only be conjectured. The arms of the camerate crinoids, which, 

 in Ordovician times, were largely formed of a single series of plates, 

 nearly all came to have a double series in the Silurian. Notwithstand- 

 ing these and other expressions of progress, there remained many rem- 

 nants of the more primitive characters, indicating that the class had 

 not yet reached its structural climax. The lower orders of the class 

 were still present, but in diminished numbers. The decline of these 

 and the rise of the higher types is in itself a mark of progress. At 

 the same time, the continued existence of the lower types teaches that 

 the introduction of the higher was not at once fatal to the welfare 

 of the lower. 



The cystoids. — Though the cystoids passed their climax in the 

 Ordovician, they continued to be abundant in the Silurian, and many 

 attained larger sizes than are known to have been reached before. 

 They show in some phases a tendency to greater regularity in the 

 arrangement of the plates, as though even they felt the influence of 

 the causes that made for order and symmetry, so well expressed in 

 the crinoids and blastoids; but on the whole, irregularity continued 

 to characterize them (Fig. 187, /, g and h). 



Other echinoderms. — The starfishes appear to have made little prog- 

 ress and to have had no large place in the fauna; the serpent stars 

 and the echinoids had even less. It is important to note these slow 

 developments of types that came to be prominent very much later, 

 for it possibly represents a general fact, viz., that great classes were 

 really slow in their evolution, however suddenly they may seem to 

 have come into existence. It is not impossible that the ascents of 

 the cystoids and crinoids to their climaxes would be found equally as 

 prolonged as those of their kin if we could trace them back through 

 their full histories. Just a little greater imperfection in the record 

 would have eliminated all trace of the ophiuroids and echinoids in 

 these early periods and would have made their appearance in abun- 

 dance at a later period seem sudden and remarkable. This suggestion 

 is not to be accepted as an affirmation, for there are some strong grounds 

 for believing that evolution was sometimes really rapid relatively. 



The continued importance of the brachiopods. — The brachiopods 



