232 THE TRANSFORMATIONS OF THE ANIMAL WORLD 



and the Placodermal Fishes appeared and vanished, 

 some like the Graptolites and the Placoderms in 

 the short space of two geological periods, or even 

 like the Clymeniae in a single stage of the Devonian 

 epoch. In the course of Secondary times we also 

 see the appearance and the disappearance of the 

 Belemnites, the Diceratidae, the Rudistse, the Ichthy- 

 osaurs, the Plesiosaurians, and the Pterosaurians, and 

 we witness the reign and extinction of several other 

 groups which appeared at the end of Primary times 

 the Spiriferidae, the Ammonites, the Stegocephalic 

 Amphibians, the Dinosaurs, the Theromorphs, the 

 Archceopteryx, etc. The Tertiary era saw the 

 commencement and the end of the true Nummulites 

 and the extinction at least of several groups of 

 mammals, the Multituberculata, the Condylarthra, 

 the Creodonts, the Amblypods, the Toxodonts, the 

 Typotherians, the Tillodonts, and among the 

 Ungulates, the families of the Hyracotheridae, the 

 Palaeotheridae, the Lophiodontidse, the Macrauchen- 

 idse, the Titanotheridse, the Chalicotheridae, the 

 Anthracotheridae, the Oreodontidae, the Anoploth- 

 eridae, the Protoceratidae, the Sivatheridae, etc. 

 We should have to multiply these cases of extinction 

 ad infinitum if we wished to enter into the details 

 of the families and genera which have entirely 

 vanished. 



However, as Abel has observed, it is expedient 

 to make reservations with regard to certain groups 

 which are only apparently extinct, but are, in 

 reality, simply transformed by evolution, at least 

 so far as some of their branches are concerned. 

 Thus it appears difficult not to seek the origin of 



