68 THE STORY OP THE 1ARTH AND MAN. 



there might be suitable abodes for coral animals. It 

 has been supposed that in the Silurian period the sea 

 might have derived some appreciable heat from the 

 crust of the earth below, and astronomical conditions 

 have been suggested as tending to produce changes of 

 climate ; but it is evident that whatever weight may be 

 due to these causes, the observed geographical condi- 

 tions are sufficient to account for the facts of the case. 

 It is also to be observed, that we cannot safely infer 

 the requirements as to temperature of Silurian coral 

 animals from those of the tenants of the modern ocean. 

 In the modern seas many forms of life thrive best 

 and grow to the greatest size in the colder seas ; and 

 in the later tertiary period there were elephants and 

 rhinoceroses sufficiently hardy to endure the rigours 

 of an Arctic climate. So there may have been in the 

 Silurian seas corals of much less delicate constitution 

 than those now living. 



Next to the corals we may place the crinoids, or 

 stone-lilies creatures abounding throughout the Silu- 

 rian seas, and realizing a new creative idea, to be 

 expanded in subsequent geological time into all the 

 multifarious types of star-fishes and sea-urchins. A 

 typical crinoid, such as the Glyptocrir.tis of the Lower 

 Silurian, consists of a flexible jointed stem, sometimes 

 several feet in length, composed of short cylindrical 

 discs, curiously articulated together, a box-like body 

 on top made up of polygonal pieces attached to each 

 other at the edges, and five radiating jointed arms 

 furnished with branches and branchlets, or fringes, all 



