440 The American Naturalist. [May, 
It remains now to ascertain if the Santacruzian fauna is 
really Eocene, or if it belongs to a more recent epoch. The 
affinities of the lower part of this formation with the Dinosaur 
beds, the presence of Creodonts and Plagiaulacidae, the 
absence of the modern groups of Ungulates, seem to me to 
demonstrate plainly that we have to do here with a fauna 
which dates at least from the commencement of the Tertiary 
epoch, and which represents the development upon the spot of 
an autochthonic fauna dating from an anterior geological epoch. 
Another proof of its antiquity is that a large number of Lara- 
mie mammals from the United States described by Marsh, 
have their nearest allies in the Eocene of Patagonia. 
On the other hand, some of the forms characteristic of the 
Puerco fauna of North America are found in beds much more 
recent (Lower Oligocene and Upper Eocene) of the Argentine 
Republic in the neighborhood of Parana (Periptychus Cope). 
It must also be said that all the authors who have studied 
the conchological fauna of the rocks of Parana (d’Orbigny, 
Darwin, Bravard, Doering, Philippi, etc.), have referred 
this fauna to the Upper Eocene or to the Lower Oligocene. 
But, between the Santacruzian fauna and the fossil fauna of 
the Parana, there is, when we consider the development of 
forms, a considerable hiatus, almost an abyss, that can only be 
filled by the aid of a half-dozen intermediate faunas yet to be 
discovered. Moreover, the greater part of the types of the 
Santacruzian fauna, considered from the point of view of an 
evolutionist, represent forms less advanced than do the totality 
of similar forms found in other continents. 
Thus all the evidences—geological, paleontological and evo- 
lutional—are in favor of the great age of the Santacruzian 
fauna. I place this fauna at the beginning of the Tertiary, 
and consider it to be contemporary with the fauna of the © 
Puerco of North America. I believe that the more we seek to 
place this fauna at a later date, the harder it will be to explain 
the affinities it presents with the faunas of other continents. 
Returning now to the fossil monkeys of this formation, I can 
pics that my brother, in his last exploration, found some neW 
Specimens, among them, a skeleton, almost complete, of Ho- 
ee 
