520 PISCES— FISHES. 



The fossil Ganoids appear in the Silurian about the same time as the 

 Elasmobranchs ; they are abundant from the Devonian to the Upper 

 Cretaceous, when the Teleosteans begin to become numerous. It is very 

 doubtful whether the primitive armoured fishes (Tremataspis, 

 Pteraspis, Cephalaspis, Pterichthys, etc.) have any claim to be con- 

 sidered as Ganoids at all. They constitute the group of Ostracodermi, 

 which, commencing in the Upper Silurian, seems to have become 

 extinct at the conclusion of the Devonian era. 



Fishes allied to the Ganoids of the present day appear in the Middle 

 Devonian, and are found in abundance until the close of the Jurassic era, 

 when they give way to the more specialised Teleostei. In Devonian 

 and Carbonifeious rocks these Ganoids may be classed in two series — 

 Crossopterygii (Holoptychiidae, Rhizodontidse, Osteolepidae, Ccelacan- 

 thidae), allied to the living Polypterus, and the Acipenseroidei (Palaeon- 

 iscidse), allied to sturgeons. But already in the Permian era we 

 begin to find representatives of that great semi-heterocercal seiies, 

 which is represented at the present day by Lepidosteus and Amia, and 

 which, in reality, passes gradually into the Physostomous Teleostei. 

 These, represented by such forms as Lepidotus, Dapedius, Eugnathus, 

 etc. , become very abundant in Jurassic rocks, while the Crossopterygii 

 and Acipenseroidei dwindle away. So does the Lepidosteid seiies in 

 the Cretaceous era ; and in Tertiary times the Ganoids were, as now, 

 nearly a thing of the past. 



Teleostei. The " Bony Fishes." 



This "order" includes most of the fishes now alive. 

 Though comparatively modern fishes, they are older than 

 was formerly supposed, as several Jurassic genera {Thrissops, 

 Leptolepis, etc.), which used to be classed as Ganoids, must 

 be considered as actual Clupeoids, or herring-like Teleostei. 

 It is, however, not until the Upper Cretaceous and Tertiary 

 epochs that they assume among fishes that overwhelming 

 preponderance in numbers which they possess at the present 

 day. The physostomous type of Teleostean is the most 

 ancient, and probably stands in a continuous genetic line 

 with the Lepidosteoid Ganoids. 



The skeleton is well ossified, with numerous investing 

 bones on the skull, others in the operculum, and on the 

 shoulder-girdle. The tail is sometimes quite symmetrical 

 or diphycercal, but in most cases it is heterocercal at first, 

 and acquires a secondary symmetry termed homocercal, for 

 while the end of the notochord in the young forms is bent 

 upwards as usual, the subsequent development of rays pro- 

 duces an apparent symmetry. The scales are in most cases 

 relatively soft. As in Ganoids, the roof of the fore-brain is 



