HISTORY OF THE MARINE FAUNA 243 



legists have attested their strong belief in the validity of the 

 argument. A very early Tertiary or late Mesozoic influx of 

 mammals from South America into North America has already 

 been alluded to. After this event long periods of time elapsed, 

 during which the two continents were seemingly separated 

 from one another. Th.en southern mammals once more 

 appeared in the north. This later invasion is proved from 

 the contents of the deposits in Texas. Here we meet 

 with gravigrade edentates, and these deposits have now 

 been definitely placed by Professor Osborn * to the middle 

 Pliocene. Hence Central America in its present form and 

 shape would be of Pliocene origin. Although Mr. Lydekker f 

 places this event at the end of the Miocene Period, Professor 

 Deperet J and Dr. Smith Woodward concur in the opinion 

 expressed by Professor Osborn which is in conformity with 

 that elicited by Professor Toula. The latter bases his evidence 

 on the fossils contained in th.e Panama and Tehuantepec 

 deposits. 



Since the testimony derived from the recent marine fauna 

 also agrees fairly well with the above conclusions, it seems 

 reasonable to conclude that Central America in its present 

 outlines, forming a highway for intercommunication between 

 North and South America, came into existence about the 

 beginning of the Pliocene Period. Thus one of the problems 

 alluded to at the beginning of the chapter is apparently 

 solved. 



Yet still another difficulty has suddenly arisen owing to 

 the reqent most surprising discovery of true edentate re- 

 mains of Megalonyx type in th,e Mascall beds of Oregon, 

 which are of Middle or Lower Miocene a,ge.|| If the 

 Gatun deposits near Panama are really, as Professor Toula 

 affirms, of Upper Miocene age, how can we reconcile the 

 submergence of Panama, and probably also of the isthmus of 

 Tehuantepec, with this latest discovery in Oregon ? The whole 

 problem is evidently much more complex than it at first ap- 



* Osborn, H. F., " Cenozoic Mammal Horizons," p. 82. 



t Lydekker, E., "History of Mammals," p. 119. 



t Dep6ret, 0., "Transformations of Animal World," p. 2S2. 



Woodward, A. Smith, "Palaeontology," p. 429. 



|| Osborn, H. F., " Age of Mammals," p. 289, 



E2 



