57 



that order. The chain of descent is carried on by the Coela- 

 canthini, the only fringe-finned ganoids occurring in the Mesozoic 

 rocks. They can be traced up from Coelaamtlms in the Car- 

 boniferous, through Holophagus in the Lias and Undina in the 

 OoUtes, up to Mucropoma in the Chalk. The family is dis- 

 tinguished by cycloid scales, hollow fin supports, and a noto- 

 chordal skeleton built on the same principle as that of the mud 

 fishes. In some genera the walls of the air bladder are ossified, 

 this peculiarity, which was first suspected by Mantell, is especially 

 remarkable in Undina and Macropoma, and can be observed in 

 sjjecimens of the latter in the Willett Chalk Collection. No 

 fossil Crossopterygids have been discovered in Tertiary strata, 

 but it is the opinion of Professor Huxley that as the rhomboidal 

 scaled Saurodipterines of the Devonian rocks are now represented 

 by the living Polypterus, so the stiff-walled lungs of the Lepido- 

 siren are the homologues of the ossified air bladder of the 

 Coelacanths, and thus that genus carried up the cycloidal branch 

 of the Crossopterygids to the present day. 



Such, in the abstract, is the life history of fishes, a class 

 characterised, like other divisions of the animal kingdom, by the 

 extinction of some groups after a brief existence, and by the 

 persistent endurance of others through untold ages. In the feAV 

 genera of living ganoids we have undoubtedly the surviving 

 descendants of a numerous and powerful race which prevailed in 

 the Devonian epoch, and by the discovery of fossil dipnoal forms 

 the progenitors of Ceratodus and Lepidosiren the Dipnoi are like- 

 \nse proved to be of ancient lineage. 



Moreover, in considering the fact that the early fishes are , 

 remarkable from a combination of diverse characteristics which 

 subsequently become the distinguishing peculiarities of distinct 

 families, and of a higher order, we find further evidence that the 

 ancient ganoids formed the parent stock from which the succeeding 

 fishes, amphibians, and reptiles have diverged. In some sauroid 

 Devonian fishes the position and structure of the teeth foreshadow 



