OP CREATION. 101 



and which being moveable, and attached to a fin, 

 enables the animal to turn itself readily on its back 

 while swimming. These spines are variously marked 

 according to the species or genus to which they 

 belong. They will be described at greater length in 

 a future chapter when treating of lias fossils. 



See then the great and striking change that had 

 supervened towards the close of this carboniferous 

 period. The corals and the encrinites remained 

 with little alteration of general form ; the trilobites 

 were nearly extinct, and seem but scantily replaced 

 by other crustaceans ; the Brachiopoda had assumed 

 new forms, which some of them retained long after- 

 wards, and which are even handed down to the 

 present day; * the ordinary bivalve and univalve 

 shells were gradually increasing ; and the prevailing 

 Cephalopoda, retaining up to this period the elon- 

 gated straight form of orthoceratites, were also de- 

 veloped in the spiral form seen in goniatites, and 

 afterwards continued in ammonites, a form better 

 fitted perhaps for the altered conditions of the sea and 

 the greater stir of life that was about to succeed. 

 But the fishes present the newest and the most 

 striking appearances. The minute, but probably fierce 

 and voracious species, which first marked the intro- 

 duction of this class of animals, had been succeeded 

 by a comparatively clumsy and awkward race, coarse 

 feeders, of small size, and indifferent swimmers, but 

 covered either with strong plaited armour or with 

 fine coats of mail, and apparently very abundantly 

 distributed. These lasted for a time, but then gave 



* Some Terebratulae of the carboniferous period are exceedingly like 

 oolitic species, and some of them closely resemble species still existing. 



