150 THE OLDEST FISHES AND THEIR FINS. 



bony fishes in ihe delicate manner observable in a bowl of 

 gold-fish. To the sharks, however, whose movements 

 largely partake of vigorous rushes, it is probable that the 

 forked modification of the fringe-tailed type is more 

 advantageous than would have been a tail of the fan 

 type. 



" Before leaving the subject of the tails of fishes, we 

 cannot forbear to mention that the alteration from the 

 fringed type, with its long central axis formed by the back- 

 bone, from each joint of which springs a pair of rays, to 

 the fan-like type, with all the rays arising together from 

 a blunt and shortened back-bone, is precisely paralleled 

 among birds. Thus the ancient birds of the Jurassic rocks, 

 known under the name of Archasopferyx, had their back- 

 bone prolonged into a long tail, from each joint of which 

 there arose a pair of feathers. Such a tail was, therefore, 

 essentially a fringed one. In modern liirds, however, as 

 we all know, the backbone extends but a short distance 

 behind the haunch-bone, and then expands into a plough- 

 share-like bone, from which the feathers of the tail radiate 

 in a fan-like manner, very similar to the rays of the tail of 

 a bony fish ; with the exception that whereas in fishes the 

 fan is placed vertically, in birds it is expanded horizontally. 

 In many groups of animals besides these we have men- 

 tioned it apjjears, indeed, that long tails have gone out of 

 fashion, as being useless incumbrances. We have instances 

 of this in the higher apes and bats, in bears, in guinea- 

 pigs, and in the naore sjsecialized kinds of flying-dragons 

 or pterodactyles. 



Having said this much as to the fins of the ancient 

 fishes, we may conclude this chapter by giving some par- 

 ticulars relating to the geological history of the Australian 

 lung-fish, which, from the structure of its fins, we have 

 already seen reason to regard as one of the most ancient 

 types of existing fishes. For a number of years there have 

 been known from the Triassic, or lowest Secondary strata 

 of Europe, fish-teeth of the peculiar type of the one 

 represented in Fig. 52. The remarkable horn-like form of 

 the ridges on these teeth suggested the name of Ceratodus 

 for the otherwise unknown fish to '^\hich they jjertained. 



