F. AmegJiino — Geology of Argentina. 15 



for me to record that, although this fauna exhibits a very small 

 number of genera in common with that of the Tertiary formations 

 of the Parana and none with the Pyrotherium fauna, it presents 

 a much greater general resemblance to the latter, and this proves 

 its great geological antiquity.^ 



The base of the Santa Cruz Formation consists of a series of 

 marine deposits of an average thickness of 30 metres, to the whole 

 of which I have given the name of the supra-Patagonian horizon 

 (etage sur patagonien),'^ because it rests directly on the Patagonian 

 Formation. That the deposits just mentioned are of a period distinct 

 from that of the latter formation, I have no doubt ; for instead of 

 Ostrea Patagonica they contain Ostrea Bourgeoisi, which is charac- 

 teristic of this horizon, and this difference seems to extend likewise 

 to almost the whole of the molluscan fauna. 



This horizon, indeed, is of great importance by reason of the 

 quantity of fossil species which it has afforded during the latest 

 explorations. According to the data which Carlos Ameghino has 

 given to me on the collections made there, in addition to Ostrea 

 Bourgeoisi, characteristic as I have said of these deposits, there is 

 also a species of the genus Crenatula whose known representatives 

 live only in the tropical regions of India and in the Ked Sea. The 

 extinct genus Amatusia is represented by a species twice as large as 

 that from the Eocene of Chili described by Philippi. There are 

 also species of the genus Hyanira, whose representatives are for 

 the most part characteristic of the Cretaceous, and more than a 

 hundred other species of shells, among which are some fresh- water 

 and terrestrial forms. There is likewise a great abundance of fossil 

 Crustacea, among which predominates the large Cancer Patagonictis, 

 Phil., which itself forms banks of a considerable extent in that 

 formation. 



I have placed all these materials in the hands of Dr. H. von 

 Jhering, who will give us the results of his studies. The foregoing 

 does not conflict with what I have been able to advance, namely, 

 that in my opinion we are dealing with an Eocene fauna, for many 

 of those shells collected to the south of the River Santa Cruz, and 

 erroneously regarded as coming from the Patagonian Formation, 

 have all been recognized as extinct species. This same horizon is 



respect to individuals they must have been more numerous than all the rodents of 

 our pampa taken together. These animals have hitherto appeared to constitute an 

 entirely extinct group without any direct affinities to any of the existing ones. Great, 

 then, was my surprise when I received a letter from the distinguished zoologist of the 

 British Museum, Mr. Oldfield Thomas, in which he informed me he had just received 

 from Colombia (New Granada) an example of a still existing genus of the group of 

 the Eocene Epanorthidse of Patagonia, a genus which the gentleman in question will 

 shortly describe. . . . Mr. J. B. Hatcher has lately informed me that the National 

 Museum of "Washington has just received an example of this same animal [ = Cceno- 

 lestes obscurus, 0. Thomas, Proc. Zool. Soc. 1895, p. 870, pi. l]. 



1 F. Ameghino, " Enum. Synopt.," etc., 1894, p. 7. 



^ Mr. J. B. Hatcher, who collected all the Cretaceous mammals described by 

 Marsh, recently examined in my house the remains of Plagiaulacoidea from Pata- 

 gonia, and told me that in his opinion they exhibit the greatest resemblance to those of 

 the Upper Cretaceous (Laramid Formation) of the United States, described by Marsh. 



