176 THE STORY OF FISH LIFE. 



ancient sharks. These are represented bj the 

 modern bony fishes the Teleostomi of science. 

 By many of the older naturalists this group was 

 sub-divided into two portions, known respectively 

 as the Ganoid and Teleostean groups. This 

 division is not followed now. 



The Teleostomi are distinguished from the 

 sharks, their ancestral kindred, by the fact that 

 both the upper and lower jaws are ensheathed 

 in bone, which in turn supports the teeth ; that 

 the skeleton is more or less well ossified ; and 

 that the gill-clefts open into a large chamber 

 with a single aperture. The outer covering of 

 this gill-chamber is constituted by a bony shield 

 known as the operculum. The outer covering 

 of the body, instead of the placoid scales of the 

 Elasmobranchii (shark-tribe), is made up of 

 a bony mosaic or of delicate horny lamellae, 

 which form the characteristic "fish - scales." 

 These are the principal characters of the sub- 

 class Teleostomi. 



The Teleostomi are divided by modern natural- 

 ists into two groups or "orders": (1) the 

 Crossopterygii or fringe-fmned fishes, and (2) 

 the Actinopterygii or ray-finned fishes. The 

 differences between these two and the more inter- 

 esting members contained in each order we will 

 now proceed to discuss. 



The fringe-finned fishes are regarded as the 

 more ancient type. Their oldest fossil remains 

 are extremely ancient, carrying us back to the 

 Devonian period. The epithet "fringe-finned" 

 is bestowed on account of the fact that in the 

 paired fins there may be distinguished two 



