54 THE GEOLOGICAL EVIDENCES 



represent a comparatively low ichthyic type, is also 

 the case with the earliest fishes, with both sharks 

 and ganoids, and not till an intermediate 

 middle period do we find a tendency on the 

 part of the tail toward homocercality. A 

 large proportion of the Jurassic ganoids are 

 provided with the modern form of tail, and these, 

 again, are preceded by a form, Semionotus, in 

 which the tail is of a distinctively transitional 

 character. 



As pertaining to the group of reptiles I can 

 present to you an equally beautiful and con- 

 clusive case of the modification of special struct- 

 ures. The crocodiles represent a fairly ancient 

 group of reptiles, beginning with the Triassic 

 period of time ; the recent genera date from the 

 period of the chalk. In their history they pre- 

 sent a remarkable series of developmental changes. 

 In the modern crocodiles, and in those of the 

 later Cretaceous period, two series of bones be- 

 longing to the roof of the mouth, known as the 

 palatines and pterygoids, are so disposed as to 



