﻿DEVONIAN PERIOD 41 



Probably their habits were not much more active than those 

 of their contemporaries, the Ostracoderms ; and their time 

 was spent more or less in proximity to the shore (Osteleopis, 

 etc.). 



Some of these ganoids possessed teeth of a complicated 

 pattern, brought about by an infolding of the surface enamel 

 (Tristichopterus). In certain forms this infolding was of an 

 extremely intricate nature (Holoptychius) ; and the teeth 

 resembled those of certain amphibians of later times. Notable 

 also is the fact that fishes with true backbones were at least 

 in promise ; the gristly spinal column in some cases having 

 undergone a certain amount of ossification (Tristichopterus). 



Other ganoids, known as the " ray-finned " (Actinopterygii) actinop- 

 were better equipped for swimming purposes, as the paired TERYGIANS 

 fins were light and flexible. Their tails, however, were of the 

 same character as those of their fringe-finned relations, nor 

 were the median fins more efficiently supported. No ossifica- 

 tion was as yet perceptible in the spinal column ; and there 

 was nothing remarkable in the structure of the teeth. These 

 undoubtedly more active fishes were apparently at the 

 commencement of their career ; and they were the " coming " 

 fishes. As yet they were few in number compared with the 

 iringe-finned ganoids ; and of no variety (Cheirolepis). 



Other fishes, probably derived from the " fringe-finned " dipnoans 

 ganoids, seem to have been forerunners of the lung-fishes or 

 double-breathers of our own time (Dipnoans). Many of them, 

 110 doubt, were denizens of the sea ; but some certainly 

 haunted lakes and rivers. The latter forms, indeed, may 

 have been true double-breathers ; i.e. with the air-bladder so 

 modified as to serve as a breathing apparatus supplementary 

 to the gills. A modification of this kind may have been 

 rendered necessary by the animals being left high and dry at 

 certain seasons of the year (Phaneropleuron, Dipterus). 



No Devonian fishes had tails of the type that now prevails. 

 In the vast majority of living fishes the tail-fin radiates 

 irom the stump-end of the spinal column. The fin, therefore, 

 is in a sense free of the backbone (homocercal tail). In the 

 case of all Devonian fishes the spinal column extended to the 

 •extremity of the animal, and its tapered end was bordered 



