332 J. B. Hatcher — Geology of Southern Patagonia. 



the ungulates described by Aineghino as from the Pyrotherium 

 beds are larger than the allied forms in the Santa Cruz beds. 

 Now as regards structure and specialization of parts we are as 

 yet unable to judge in many cases just which forms are the 

 more specialized. In not a few instances, in his descriptions 

 of remains from the Pyrotherium beds, he shows that they are 

 not distinguished either generically or specifically from allied 

 forms in the Santa Cruz beds, sometimes that they are 

 decidedly more specialized than the latter, and almost always 

 that they are of a size and structure showing a close relation 

 with the fauna of the Santa Cruz beds and not at all what we 

 should expect from the Cretaceous. As instances of this I may 

 cite that in his " Premiere Contribution a la Connaissance de 

 la faune Mammalogique des Couches a Pyrotherium " on page 

 44, in closing his description of Asmodeus Osborni he remarks, 

 " Cet animal est assurement un des plus gros mammiferes qui 

 ait foule la surface de la terre." Again on page 50 in defining 

 the genus Ancylocwlus, he compares it with Homalodonto- 

 therium, a closely allied genus from the Santa Cruz beds but 

 which is distinguished from the latter partly by the dental 

 formula, which he finds to be I|CiP|-M| in Ilomalodonto- 

 theriwn, while in Ancylocoelus from the Pyrotherium beds 

 there is a reduction in the number of inferior premolars to three 

 on either side, a marked advance over the Santa Cruz form. 

 On page 56 he mentions edentates from these beds of the 

 stature of Mylodon. Many other similar examples might be 

 cited, but enough has been done to show that we are not deal- 

 ing with a Cretaceous fauna. Thus, from his own figures and 

 descriptions it would appear that in the matter of size at least, 

 there is a decided advantage in favor of forms found in the 

 Pyrotherium beds as compared with related forms from the 

 Santa Cruz beds. In the history of the development of every 

 mammalian phylum in the northern hemisphere, so far as I am 

 aware, there is a decided and gradual increase in the size of 

 the individual from the lower to the higher forms. According 

 to Dr. Ameghino, exactly the opposite has taken place in South 

 America. It would be interesting to know why it is that 

 natural causes always working by the same methods have pro- 

 duced such opposite results in the two hemispheres, when as is 

 everywhere shown, especially among the ungulates there are 

 such marked cases of parallelism in structural development. 



Whatever may be the relation of the Pyrotherium beds to 

 the Santa Cruz beds, I feel sure that the mammalian fauna 

 described by Florentino Ameghino as from the Pyrotherium 

 beds does not occur associated with the Dinosaurian remains 

 of the Guaranitic beds, unless such association is due to sec- 

 ondary deposition of the latter or a superficial mingling of 



