LATER LIFE ON THE EARTH 41 



Brachiopoda ; while the Cephalopoda maintained a 

 very important position up to the close of the 

 Mesozoic era, and then collapsed. 



The subject naturally divides itself into three parts 

 Deutozoic, Mesozoic, and Uainozoic. The 

 Deutozoic opens with a great development of fishes ; 

 the Mesozoic with the expansion of reptiles ; and the 

 Cainozoic with that of mammals and birds. The 

 passage between the Deutozoic and the Mesozoic is 

 not marked by the rapid extinction of any group, 

 because the new development of life was on land, 

 and did not interfere with the fishes of the ocean. 

 But the great expansion of mammals and birds was 

 coincident with the extermination of the huge 

 Mesozoic reptiles of the land, the sea, and the air; 

 so that the break between the life of the Cretaceous 

 and that of the Eocene is greater than that between 

 any other two consecutive periods. 



Deutozoic Life. The two great features of the 

 life of the Deutozoic era are the abundance of land 

 vegetation and the first appearance of air-breathing 

 vertebrates; but, before saying more about them, 

 we will first take a glance at the fishes. 



The armour-plated Ostracodermi of the Silurian 

 became numerous in the Devonian. These curious 

 animals are of very uncertain affinity, and by some 

 naturalists are excluded from the class of fishes 

 altogether, and placed alongside the Lampreys ; 

 because, like them, they had no low r er jaws; while 

 Mr. Traquadr considers them to be true fishes, 

 descended from some primitive Elasmobranch. 

 Their internal skeleton was entirely cartilaginous, 



