14 F. Anieghino — Qeohgy of Argentina. 



These primitive characters are recognized by Lydekker himself, 

 who directs attention to thera in a very special manner, and thus 

 I need not enter into details. 



The Santa Cruz Formation". 



Above the Patagonian horizon there comes another very distinct 

 formation, consisting of a series of marine and terrestrial deposits, 

 the latter predominating over the former, to the whole of which 

 I have given the name of Santacrnzian.^ Observed first in the 

 interior, where the Patagonian Formation does not appear, it was 

 supposed originally to be much anterior to this, but the subsequent 

 exploration of other regions in which both formations are exhibited 

 superposed has demonstrated the contrary, namely, that the Santa 

 Cruz Formation rests on the Patagonian Formation.'^ The circum- 

 stance that the Tertiary deposits of the Parana had been identified 

 with the Patagonian Formation, and the fact that the mammal fauna 

 of the Santa Cruz Formation is evidently of a more ancient aspect 

 than that of the Parana, doubtless contributed to the commission 

 of that error. Unfortunately, the authors who have identified the 

 marine formations of the Parana with the Patagonian Formation 

 have fallen into an error, since the latter is of much older date. 



On the Atlantic coast, the Santa Cruz Formation appears to the 

 south of the River Santa Cruz, gradually increasing in thickness 

 in proportion as there is an attenuation of the Patagonian Formation, 

 which finally disappears completely below the sea-level before 

 reaching the EiverCoyle; between this point and the River Gallegos 

 the maximum thickness of the formation is about 260 metres. Tlie 

 greater part, or the Santacruzian zone properly so-called, which is 

 uppermost and approaches a maximum development of about 200 

 to 230 metres, is of terrestrial or fresh- water origin; this consists of 

 a series of strata of clay and volcanic detritus, sometimes also with 

 calcai'eous accumulations, containing almost everywhere numerous 

 remains of extinct mammals, gigantic birds, and also, though in less 

 quantity, remains of reptiles. The mammal fauna is one of the 

 most remai'kable claiming the attention of all natui'alists occupied 

 with the study of the vertebrata. It is not necessary, and this is 

 obviously not the place, to enter into the details of the subject, for 

 the fauna in question has been the subject of numerous publications, 

 and much more will continually be published upon it.^ It sufiices 



' F. Ameghino, " Contrib. Conoc. Mamif. Fos. Repub. Argent.," pp. 16, 17 

 (Buenos Aires, 1889). 



^ F. Ameghino, "Enum. Synopt. Mamm. Foss. Eoc. Patagonia," pp. 4, 5 

 (Buenos Aires, 1894). 



■* I cannot, however, omit to mention here a recent discovery which has not yet 

 been published, and which is doubtless destined to rouse great interest in the zoo- 

 logical world. One ot the most singular groups of mammals of the Santa Cruz 

 Formation comprises a series of small animals with the dentition somewhat similar 

 to that of the kangaroos of Australia, but still more so to that of the fossil Plagi- 

 aulacoidea of the Mesozoic formations of Europe and North America. Those 

 animals, marsupial though not syndactylous like the kangaroos, which I have 

 distributed into the four families of Abderitidfe, Decastidte, Epanorthidae, and 

 Garzonidoe, were at this period extremely numerous in genera and species, and in 



