i6 



Vertebrata. 



column, and sometimes appears as a simple uniform 

 fringe evenly distributed around the pointed vertebraj 

 axis, or else the whole caudal area of the vertebral 

 column becomes upturned and 

 the tail fin forms a large lobe 

 on the under surface of the 

 axis while simply margining the 

 end of the caudal vertebrae ; 

 such a tail is spoken of as an 

 unequally lobed tail. When, as 

 in bony fishes, the extremity of 

 the spinal column becomes con- 

 verted into a single bone, then 

 the fin borne by it is usually 

 an evenly bilobed tail, such as 

 that of a herring or salmon 

 or sole. It is, however, an in- 

 ( tere sting fact in relation to this 

 that the young fry of the salmon 

 or other bony fish has origi- 

 nally a tail of the unevenly 

 lobed character, which by the 

 ' shortening of the upper part, 

 and the expansion of the lower 

 lobe, becomes even, as we find 

 it in the adult. 



In the simplest fishes the 

 notochord persists through life, 

 and such fishes have generally a uniformly fringed 

 tail, as in the lampreys. 



The structure of the skull differs in the various 

 subdivisions of the class : in some it is a simple carti- 



