LATEE LIFE ON THE EAETH 43 



takes place chiefly in radiating plates, appeared in 

 the Carboniferous period, and at once became com- 

 mon ; but they had no sharp teeth in the jaws, only 

 blunt crushing teeth on the palate. The rays, in 

 which ossification of the vertebrae takes place chiefly 

 in concentric plates, although first known in the 

 Carboniferous, remained rare for a long time, and 

 only became abundant in the Cainozoic era. But 

 the most interesting group of Elasmobranchs is the 

 Acanthodii, in which the basal cartilages of the 

 pectoral fins are much shortened ; the rays are 

 arranged like those of ordinary fishes, and the scales 

 are enamelled. These became extinct at the end of 

 the Deutozoic era. 



The sub-class Teleostomi, which contains all the 

 bony fishes, appears first in the Devonian as Crossop- 

 terygians, with their paired fins formed by a long 

 segmented axis, fringed on both sides with rays, like 

 the vane of a feather. They attained their maxi- 

 mum in the Devonian, and then slowly declined, 

 being at the present day represented only by 

 Polypterus of the rivers of Africa. A little later 

 came the Actinopterygians , in which the paired fins 

 are supported by numerous radiating rays, as in the 

 Acanthodii ; and in the Carboniferous period they 

 became common. All the Deutozoic Teleostomi had 

 cartilaginous skeletons, enamelled scales, and either 

 diphycercal or heterocercal tails. 



The Dipnoi, or lung-fish, breathe air as well as 

 water. Remains of them have been found in 

 Devonian rocks; but, as their teeth are the only 

 hard parts of the skeleton , very little is known about 



