HISTORY OF THE MARINE FAUNA 
243 
logists 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. Then 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 j 
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 the 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 recent most surprising discovery of true edentate re¬ 
mains of Megalonyx type in the Mascall beds of Oregon, 
which are of Middle or Lownr Miocene age.|| 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, R., “History of Mammals,” p. 119. 
f Bep6ret, C., “Transformations of Animal World,” p. 2S2. 
§ Woodward, A. Smith, “Palaeontology,” p. 42y. 
|| Osborn, H. F., “ Age of Mammals,” p.289. 
R 2 
