﻿SILURIAN PERIOD 



29 



GRAPTOLITES 



CYSTIDS 



comb appearance, were conspicuous in this activity (Favo- 

 sites). The body-cups, although of thick texture, were per- 

 forated ; and a community of life was, therefore, to a certain 

 extent established. Other colonies, presenting a chain-like 

 appearance, were also much to the fore (Halysites) ; but 

 individualism on these reefs was more or less rigidly main- 

 tained. The two schools of compound corals — communicative 

 and close-walled — were, therefore, represented. 



Graptolites were now on the wane. The more populous 

 double-rowed colonies seem to have died out in the early part 

 of the Period. The colonies with single rows of individuals 

 continued somewhat longer. But though their wants were 

 fewer, they were having a hard struggle for existence ; and 

 by the close of the Period they became practically extinct. 



Changes had taken place in the ranks of the cystids. 

 The progressive Ordovician families — characterised by a 

 more symmetrical arrangement of the box-plates, and by the 

 development of " arms " — were still in existence : and the 

 families which had shown progress only by developing 

 "arms" were still living, but in diminished numbers. The 

 more primitive Cambrian family, without " arms," and with 

 irregularly arranged plates, had apparently died out (Aristo- 

 cystidcz). Stems were but little, if at all, developed by early 

 cystids, but in the Silurian Period some well-stemmed forms 

 were to be seen. These more dignified creatures had also other 

 advantages. Their facilities for collecting food were increased 

 — as in the case of some crinoids — by the development of 

 feathery appendages on the somewhat diminutive arms : 

 and they also possessed improved channels for conveying 

 food to the mouth (Lepadocrinus). 



Cystids seem to have been fairly prosperous during the 

 Silurian ; but there were signs of decline. They were, in fact, 

 slowly failing in their competition with crinoids or " sea- 

 lilies," and had, indeed, by this time, been far surpassed by 

 the latter alike in number and variety. 



Crinoids were spreading in the seas with all the luxuriance sea-lilies 

 of tropical jungles ; and many large-cupped, graceful forms 

 were now to be seen (Periechocrinus). Those with mouths 

 rendered more free for the reception of food by the removal 



