HISTORY OF THE FOSSIL REMAINS. 47 



marking the outgoing and the incoming of the two most re- 

 markable phenomenal aspects of the physical history of our 

 planet — and the products of which, garnered up in ages long 

 past, are now so essential to our progress in art, science, and 

 commercial enterprise. 



The fishes up to this period are all of a type, and are modelled 

 according to a special plan, both in the scaly envelope and 

 osseous structure. The interior skeleton is composed of car- 

 tilage, and the outward covering of true bone, the scales, fins, 

 tail, and external plates of the head being all constructed of 

 bony material. The back-bone, or dermal ridge, extends to the 

 extremity of the tail-fin, thus at once adding strength to the 

 cartilaginous body, and giving a greater degree of propelling 

 power to its caudal organ. It is hence called Heterocercal, 

 in contrast with all existing fishes whose back-bone terminates 

 within the tail-fin, and is therefore termed the Homocercal, or 

 equally-lobed structure. The existing ray, shark, and sturgeon 

 are exceptional cases. Hence in these and in some other 

 respects, especially in their dentition, Agassiz remarks that the 

 fishes of this period all partake, more or less, of the characters 

 which, at a later time, are exclusively found in reptiles, and no 

 longer belong to the fishes of the present day. They are there- 

 fore classed among the Order of Sauroids, to denote their rep- 

 tilian affinities ; and thus Agassiz considers that " the earliest 

 fishes are rather the oldest representatives of the type of ver- 

 tebrata than of the class of fishes, and that this class assumes 

 only its proper characters after tlie introduction of the class of 

 reptiles upon earth." * 



It should be the great aim of the geologist, in following out 

 these generalizations, to ascertain with more and more pre- 

 cision the true affinities of the groups of the same zone and 

 period to one another, and to those especially of the preceding 

 and following epochs ; and thus to note the character of the 

 successive changes the organic kingdom has undergone, the 



* EssHij on Classijicadon, p. 110, by M, Agassiz. 



