OF EVOLUTION. 55 



form the boundaries of the posterior nostrils ; 

 in the crocodiles that preceded these, or those 

 of the early Cretaceous and Jurassic periods, only 

 the palatines are produced to form these nares; 

 and in the still earlier and earliest forms, those 

 of the Triassic period, neither the one bone nor 

 the other is concerned in the structure of the 

 parts in question. Correlatively with these changes 

 other modifications, scarcely less significant, mark 

 the rise of this very remarkable animal group. 

 Thus, the earlier crocodilian forms retain a 

 primitive character in the biconcave form of the 

 vertebrae a structure belonging primarily to the 

 lowest group of vertebrates, the fishes. This 

 structure is replaced in the Cretaceous period 

 by the cup-and-ball, or proccelous vertebra, which 

 is also the type of the Tertiary and modern 

 forms. 



Other instances of similar variation and pro- 

 gression could readily be cited, but my limited 

 time will only permit me to dwell upon a few 

 very striking cases drawn from the class of 



