SUCCESSIVE MAMMALIAN FAUNAS 225 



one is the flat-horned and the other the twisted-horned or 

 strepsicerine type, such as is illustrated by the Eland and Kudu 

 of modern Africa. The latter may, however, be related to 

 the peculiarly North American Prong-Buck (Antilocapra) 

 and not to the strepsicerine antelopes of the Old World. The 

 last survivors of an exclusively North American family, the 

 foreodonts, which were wonderfully numerous and varied 

 from the upper Eocene onward, are found here. 



The fmastodons ('\Gomphotherium) of this formation had 

 well-developed tusks in the lower as well as in the upper jaw, 

 and in one species the chin-region or symphysis of the lower 

 jaw was greatly prolonged, an ancient feature. 



That the South American edentates had already reached 

 the northern continent is sufficiently proved by remains of 

 fground-sloths, which are, however, too incomplete to permit 

 identification of the genus. fGlyptodonts have not yet been 

 found, but this fact does not demonstrate that they had not 

 accompanied the fground-sloths in their migration, for at no 

 time did they range so far north as Nebraska or northwestern 

 Nevada, and the only mammal-bearing formation of lower 

 Pliocene date known in the south, the Alachua Clay of Florida, 

 has yielded too scanty a list of fossils to make its negative 

 evidence at all conclusive on this point. 



The mammals of the middle and especially of the lower 

 Pliocene were much stranger and more primitive than might 

 be inferred from the foregoing brief account. Except several 

 of the Rodentia and perhaps one or two of the Carnivora, the 

 genera are all extinct and such familiar terms as horses, rhinoc- 

 eroses, camels, etc., can be employed only in a very compre- 

 hensive sense, as equivalent to families. 



The Pliocene of South America is involved in some obscurity ; 

 not that there is any question as to the formations, or their 

 order of succession, but there is much doubt as to the limits 

 of the epoch both above and below. The latest Pliocene 

 fauna, that of the Tarija Valley in Boh via, was essentially the 



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