432 Bulletin American Museum of Natural History. [Vol. XXXIV, 
Arctostylops and the Isotemnid@ there is no very wide gap in time; between 
the Palaanodonts (Metacheiromyide) and the Loricates there is a very 
wide gap, sufficient for a great deal of divergent evolution. 
The interpretation of this discovery depends upon the interpretation 
of the occurrence of Metacheiromys. If the latter be regarded as a relict of 
a formerly northern distribution of the Edentata, the same explanation 
will apply to Arctostylops. Ii it be regarded as an immigrant from South 
America, then this little Homalodothere may have arrived in the same way. 
A thorough revision of the Paleocene faunze with the new material recently 
acquired may enable us to recognize possible or probable sources for the 
LEONTINIA 
PLEUROSTYLODON 
NoOTvTOsTYLOPS 
ARCTOSTYLOPS 
Fig. 3. Arctostylops steini, type specimen, compared with South American Entelonychia. 
Crown views of lower teeth. 
South American faune, the oldest of which, that of the Casa Mayor horizon, 
is regarded by Schlosser as probably Middle or Upper Eocene. Schlosser’s 
opinion on this point is entitled to especial respect as he has obtained and 
studied large collections from the older Tertiary horizons of Patagonia. 
Professor Scott, in his recent volume, regards the fauna as Eocene, but does 
not attempt any more precise correlation. I had formerly (1902) regarded 
the Notostylops fauna as Paleocene, but further consideration of the faunal 
evidence led me to place it later, and I am now disposed to agree with 
Dr. Schlosser’s estimate. If this correlation be correct, we may find among 
the unspecialized trituberculate placentals of the Paleocene faunz types 
which will serve as a source for the various specialized groups peculiar to 
the South American Tertiary. But we cannot consider the latter as directly 
