42 THE GEOLOGICAL EVIDENCES 



histories of the primary and secondary groups 

 of the larger divisions of the animal kingdom, 

 we meet with a repetition of much the same 

 order of appearance. The fishes, for example, are 

 represented in the oldest formations exclusively 

 by such forms as betray a comparatively low 

 grade of organization ; these are the sharks and 

 ganoids, in which the vertebral column remains 

 largely in the embryonic condition, becoming only 

 partially ossified in most cases. The lung-fishes, 

 which are a direct modification of the ganoid 

 type, representing, however, a considerable amount 

 of specialization in the development of a respir- 

 atory apparatus adapted to breathing directly the 

 oxygen of the atmosphere, appear considerably 

 later, possibly in the Carboniferous period, but are 

 already preceded by an intermediate type, that 

 of the dipteroid ganoid. The more highly organ- 

 ized fishes, the teleosts, or bony fishes, appear for 

 the first time, as far as we know, in the deposits 

 of the Cretaceous period, and may consequently 

 be looked upon as a comparatively modern group ; 



