EVOLUTION OF THE PROBOSCIDEA. 



101 



number of Proboscideans of such diverse forms, points to the conclusion that they were 

 a predominant Eocene type in this region, and that probably many other peculiarly 

 modified members of the group are still to be found. Furthermore, from the presence 

 of a gigantic form like Barytherium in the Middle Eocene beds, it seems clear that 

 still earlier types are to be sought in the Lower Eocene. 



Of the genera mentioned above, Palceomastodon and Moeritherium appear to be on 

 the direct line of the ancestry of the Elephants and Mastodons, and will be described 

 below. Arsinoitherium, if indeed it is a Proboscidean at all, is an extraordinarily 

 specialised and aberrant type, and appears to have left no descendants. Barytherium 

 shows some similarity to the Dinotheriidse in the structure of its teeth, and may be 

 nearly allied to those animals, though probably not their direct ancestor. Barytherium 

 also shows certain likeness to those remarkable South American mammals, the 

 Pyrotheriidse, which have been described by Ameghino, who regards them as 

 Proboscideans of Cretaceous age. This writer has lately published a paper* in which 

 he seeks to trace back the Proboscidea through a long series of genera to a group of 

 Jurassic mammals, the Microbiotheriidse, apparently closely allied to the Didelphiidre. 

 While it seems possible that some of the forms described by Ameghino are actually 

 Proboscideans, and that there was a land connection between Africa and South 

 America in older Tertiary times, as for various reasons has been considered 

 probable by many writers, nevertheless it is difficult to accept his views as to the 

 age of the beds in which these remains occur, and it seems far more likely that the 

 Pyrotherium-beds are Tertiary rather than Cretaceous. Ameghino further considers 

 that Moeritherium and Barytherium form side branches and are not on the main 

 line of descent of the later elephants. The whole question of these South American 

 forms is at present so obscure that, for the purposes of the present paper, they may 

 be ignored and attention directed to the Old- World genera only. 



In following out the course of the evolution that has taken place in a group of 

 mammals we may either begin with the later more specialised forms and then pass 

 backward to the earlier and simpler members of the series, or the reverse. In the 

 present communication it is proposed to adopt the former alternative, since in this 

 case it is particularly interesting to note the persistence with which modification along- 

 certain lines has taken place and to show that by following back in time the known 

 Post-Tertiary and Neogene species it would have been almost possible to predict that 

 the earlier terms of the series would present precisely the character which we now find 

 actually existing in the Middle and Upper Eocene genera, Moeritherium and 

 PalcBomastodon. 



It is not now intended to enter into details of the evolution of the group even 

 were this possible, but merely to take four stages, so as to show the general trend of 

 the modification that has taken place. The numerous intermediate stages between 



* " Linca Filogenetica de los Proboscideos," ' Anales del Museo Nacioual de Buenos Aires,' Tom. VIII 

 (Ser. 3, torn. 1), 1902, p. 19, d seq. 



