64 EVOLUTION IN THE PAST 



skull, and in some cases as regards the limbs (Cynognathus, 

 Tritylodon, Theriodesmus). 



(CHELONIANS) Other reptiles appear to have been developing on tortoise 

 lines, and were protected on the anterior part of the back by 

 several bony plates (Diadectes, Cacops). In some of the 

 smaller forms the plates seem to have covered the whole 

 back ; and, moreover, to have curved down on either side, 

 affording the animals an almost box-like protection (Oto- 

 cceeus, Conodectes). These creatures, it is clear, were intent 

 on a quiet life. The world was becoming troublous and 

 strifeful ; and a walled existence afforded some escape from 

 surrounding evils. 



By the evolution of amphibians, fishes had lost the leader- 

 ship of life, and the rise of reptiles placed them still farther 

 from the van. In spite, however, of their humiliation, their 

 evolution was proceeding with some rapidity. Fish-life, in 

 fact, had never lagged ; indeed, so far as evidence goes, 

 it had come on with leaps and bounds from the time of its 

 first appearance. But in spite of rapid development, fishes, 

 at the close of the Carboniferous Period, had not undergone 

 much ossification of the internal gristly skeleton, and so fell 

 far short of the great majority of living fishes. Bony sub- 

 stance they certainly possessed, but this was more in the 

 hard scales than in the internal framework. 



ACTINOP- It is in this Period that a glimpse is gained of some flexibly 

 TERYGIAN finned ganoids with internal skeletons that had in great part 

 GANOIDS become ossified (Acentrophorus). This improvement had 

 probably been effected at the expense of the hard scaly 

 covering. These fishes, moreover, had their median fins 

 more adequately supported than was the case with their 

 forerunners ; and the tail was losing its old unsymmetrical 

 (heterocercal) character. The upper lobe, hitherto much 

 elongated by the invasion of the spine, was becoming spine- 

 less ; and the short lower lobe was lengthening. The two 

 lobes were thus becoming symmetrical, and the end of the 

 " backbone " was being modified so as to provide them with 

 a common base. In other words, the tail was becoming of 

 the homocercal type, such as characterises the vast majority 

 of fishes now living. 



