294 GEOLOGICAL CONTEMPORANEITY- ix 



older and the younger Fishes ; the former, the 

 Devonian Ganoids, being almost all members of 

 the same sub-order as Polypt&nes, while the 

 Mesozoic Ganoids are almost all similarly allied 

 to Z/epidosteus. 1 



Again, what can be more remarkable than the 

 singular constancy of structure preserved through- 

 out a vast period of time by the family of the 

 Pycnodonts and by that of the true Ccelacanths : 

 the former persisting, with but insignificant 

 modifications, from the Carboniferous to the 

 Tertiary rocks, inclusive; the latter existing, 

 with still less change, from the Carboniferous 

 rocks to the Chalk, inclusive ? 



Among Eeptiles, the highest living group, that 

 of the Crocodilia, is represented, at the early part 

 of the Mesozoic epoch, by species identical in the 

 essential characters of their organisation with 

 those now living, and differing from the latter 

 only in such matters as the form of the articular 

 facets of the vertebral centra, in the extent to 

 which the nasal passages are separated from the 

 cavity of the mouth by bone, and in the pro- 

 portions of the limbs. 



And even as regards the Mammalia, the scanty 

 remains of Triassic and Oolitic species afford no 

 foundation for the supposition that the organisa- 



1 " Memoirs of the Geological Survey of the United Kingdom. 

 Decade x, Preliminary Essay upon the Systematic Arrange- 

 ment of the Fishes of the Devonian Epoch." 



