OF EVOLUTION. 19 



just given you; it must sooner or later show that 

 in the period intervening between the first appear- 

 ance of fishes and the earliest development of 

 amphibians there existed a type of fish more 

 closely related to the amphibian than the ordinary 

 fishes in other words, a connecting link more or 

 less closely related to the mud-fishes. Such a form 

 we find in Dipterus and its allies, fishes that belong 

 to the Devonian period of time ; and if any proof 

 were further wanted indicating the antiquity of 

 the existing group of lung-fishes, we have but to 

 point to the occurrence of one of our modern 

 genera, Ceratodus, already in the deposits of the 

 Permian period. Ceratodus, in fact, represents the 

 oldest living vertebrate type known to naturalists. 



There is a remarkable structural peculiarity 

 belonging to a very large number, if, indeed, not to 

 the vast majority, of the earliest amphibians, which 

 seems to distinguish them from all the modern 

 members of the same group of animals. This is a 

 singular labyrinthine infolding of the substance of 

 the teeth, which has given to the group the name 



