F. Ameghino — Geology of Argentina. 9 



Notwitlastanding the discovery of these animals of small size, the 

 ungulates always maintain the same numerical preponderance, the 

 genera and species being those which I indicated in my preliminary 

 memoir on this fauna, though to the known types there are now 

 added other new ones still more interesting. There has appeared, 

 for example, a whole series of ungulates with mammillated teeth, 

 the classification of which is for the moment very doubtful, but 

 which, at least in the dentition, appear to resemble the Phenaco- 

 dontidse of North America ; for the present the onl}' notable difference 

 worthy of remark is that the Argentine forms exhibit the dentition 

 in a continuous series. 



As I have already suspected previously and pointed out in my 

 work on this fauna, the Proterotheriidse, such as are known from 

 Santa Cruz and from the deposits of the Parana, do not exist in 

 this formation. The forms which replace them have a complete 

 dentition, presenting also, at least in the shape of the molars, 

 a certain resemblance to the Ancylopoda, those ancient ungulates 

 with hook- shaped toes and the ungual phalanges cleft as in some 

 edentates. Moreover, it happens that the animals which represent 

 the predecessors of the Macraucheniidse in this formation also 

 exhibit affinities with the Ancylopoda. Among the Typotheriidae 

 the same resemblance is observed, while some forms apparently of 

 this group, from the structure of the skull and the aspect of the 

 dentition, seem to be the direct forerunners of the modern Hyra- 

 coidea of Africa and Asia, one of the few types of existing ungulates 

 which has hitherto appeared completely isolated and without 

 predecessors. 



In the representatives of the order of Toxodonts, which are 

 very numerous, these same transitional characters are still more 

 surprising ; for while, on the one hand, they pass into the Ancylopoda, 

 on the other they approach the typical Perissodactyla (Stereopterna) : 

 this transition is very clearly seen both in the conformation of the 

 dentition and cranium and in the construction of the foot. The 

 transition between the Ancylopoda and the Astrapotheria, at least 

 in the dentition, is equally complete. 



These various groups show among themselves less differences 

 than in the Santa Cruz Formation, while by the characters which 

 they exhibit in common in the Pyrotherium Formation, they seem 

 to converge towards the Ancylopoda as if this were the original 

 trunk of the ungulates. The only group which appears as yet to 

 constitute an exception is that of the Pyrotheria and its descendants 

 the Proboscidea. 



However, perhaps the most important discovery is the finding 

 in these same deposits of remains of monkeys perfectly characterized. 

 We are not, indeed, dealing with types as specialized as the 

 Homunculida3 of the Santa Cruz Formation, but with lower forms 

 which, although they belong indisputably to the group of monkeys, 

 pass into a whole series of animals which have been arranged 

 among the ungulata. 



In an article on the fossil monkeys of the Eocene of the Argentine 



