GENERAL STRUCTURE. 



921 



kpididce, which are usually placed in the former, should not be 

 transferred to the latter group ; and we may thus confidently expect 

 to find a complete transition between the two. Although many of 

 the Cretaceous Teleostei are more or less closely allied to existing 

 types, it is not until the Eocene that we find a fish-fauna compar- 

 able to that of the present day ; and we may note that the resem- 

 blance of the fishes of the Eocene to those now living is in marked 

 contrast to what obtains in Mammals, where the majority of Eocene 

 genera are extinct. The persistence of some genera of Fishes 

 throughout long geological epochs is indeed a noteworthy cir- 

 cumstance, and is nowhere more marked than in the case of 

 Ceratodus, which has lived on continuously from the Triassic period 

 of Europe, and also from that of certain North American beds, 

 which are usually referred to the Permian. 



In regard to the origin of the various orders of Fishes, it is pretty 

 evident that the Teleostei were derived from the Ganoidei, and that 

 the Dipnoi were closely related to one branch of the latter. The 



Fig. 847.— Conodonts, greatly enlarged, a, b, Acodjts acutus : c, D, Prionodus elegans ; e, 

 An unnamed form ; a-d, From the Silurian of Russia ; e, From the Carboniferous of North 

 America. (After Pander and Newberry.) 



phylogenetic relationship of the Ganoids to the Elasmobrancheans 

 is, however, still unsettled, although recent researches tend to show 

 a close connection between the more primitive and least specialised 

 groups of the two orders. Palaeontology has not, indeed, yet taught 

 us from what group of animals these primitive Ganoids and Elasmo- 

 branchs were themselves derived. It has, however, been suggested 

 by Professor Cope that the Placodermoid Ganoids were closely re- 



