48 THE STORY OF FISH LIFE. 



from those of ourselves in several particulars, 

 but the differences are rather those of degree 

 than of kind. In the dog-fish we should find 

 the upper jaw quite distinct from the skull, 

 and connected therewith only by ligaments. 

 Attached to the hinder end of this jaw is the 

 lower jaw. In the very young dog-fish these 

 two formed one piece, and only later became 

 jointed to form separate jaws. In ourselves 

 the upper jaw is firmly fixed to the skull, not 

 separate as in the dog-fish. The next pair 

 of arches serve for the support of the tongue ; 

 the succeeding arches for the support of the gills. 



This form of skull — a cranial box for the 

 lodgment of the brain and organs of hearing, 

 sight and smell, and a series of arches embedded 

 in the upper region of the mouth and gullet — 

 represents the simplest of all types, and is the 

 starting-point in the study of the skulls of all 

 other vertebrated animals. Let us now rapidly 

 sketch the outlines by which the complex com- 

 bination of bones, with which we are more or 

 less familiar, came into being. 



An examination of the head of a sturgeon, 

 which has been carefully dissected, would show 

 that it differed in no important respects from 

 that of the dog-fish. But, if we turned to a 

 freshly killed sturgeon, we should meet with a 

 very great surprise. In the first place it would 

 be found that the head was not covered by a 

 " shagreen-like " skin as in the dog-fish, but by an 

 exceedingly dense bony armour. Secondly, this 

 armour, when closely examined, would be seen 

 to be made up of a number of separate and 



