Boundary in South America. 47 



opinion that the Early Tertiary fauna of the Antarctic 

 regions had not been preserved. This opinion could be 

 maintained, as long as the San Jorge-fauna was imper- 

 fectly known. Now, on the basis of the results of this 

 study, I do not hesitate in declaring that the San Jorge- 

 fauna, as a fauna of Antarctic origin, is identical with 

 the hitherto missing fauna of the Early Tertiary. Not 

 only the diastrophic events, but also the fossils charac- 

 terize the San Jorge-Formation as a forerunner of the 

 Patagonian Formation. It therefore lies at the base of 

 the Tertiary. 



On the other hand, the comparative studies of Wilckens 

 proved, that there exist certain relations between the 

 gastropoda and pelecypoda of the San Jorge and the 

 Upper Senonian of South Patagonia and Antarctica. 

 These correlations are not only indicated by some com- 

 mon species, but also by the general character and the 

 generic composition of both faunas. This fact may be 

 appreciated as evidence, that in the regions at the south 

 of the present South America there existed during sev- 

 eral geologic periods, a prominent center of evolution 

 for marine faunae. This biological center maintained 

 its character with peculiar tenacity. It gave to the 

 northward invading transgressions new forms continu- 

 ously creating varieties, modifications and species, but 

 always maintaining the essential character of the original 

 fauna. 116 In this well-defined sense it may be justifiable 

 to suppose a gradual transition from Cretaceous to Ter- 

 tiary. Weighing the participation of different elements, 

 there is a remarkable preponderance of the Tertiary 

 element in the San Jorge-fauna. The entire absence of 

 ammonites and baculites may be considered as the most 

 important fact in this sense. 



Unfortunately, the slight relations of this fauna to 



116 Two examples may be cited to illustrate the isolated evolution and 

 persistence of certain forms in this region. The genus Tylostoma (Pseudo- 

 tylostoma v. Ihering) is already represented in the Lower Cretaceous (Bar- 

 remian) of Neuquen by a species T. aequiaxis Coq. The Tylostomas in the 

 Tertiary of this province may be regarded as direct descendants of that 

 form. Further, the so-called Exogyra ostracina Lam. var. rnendozana Ih. 

 (which I regard as a new form) is very probably a descendant of Ostrea 

 minos, a form which also appears in the Lower Cretaceous of Neuquen. 

 A future revision of the San Jorge will certainly add many more examples 

 of this kind. Such a revision will show a continuous evolution of this 

 molluscan fauna, which' originally came from Upper Jurassic-Cretaceous 

 elements of the South Andine province (see Uhlig, 1. c.) and gradually 

 developed itself into a characteristic fauna of the Antarctic regions. In 

 spite of this gradual evolution, the faunistic break between the Cretaceous 

 and Tertiary cannot be clearer than it is in this province. 



