ORIGIN AND DISTRIBUTION OF MAMMALS 307 



the Folydolopidae are extinct, and replaced to some extent by the first 

 Caenoleslidae — Palaeothentes, Farabderites and Halmarhiphus. The first immi- 

 gration of rodents — Cephalomys and Eosteiromys — is especially noteworthy. 



Intercalated between this fauna and that of Santa Cruz, both strati- 

 graphically and morphologically, is the fauna of the Colpodon beds, which, 

 according to Tournouer, are especially well developed at Lake Coli Huapi. 

 Ameghino enumerates from it the very doubtful primates — HomuncuUtes and 

 Pithecidites — new types of rodents — Acaremys, Spaniomys and Perimys ; among 

 edentates, Propalaeohoplophorus ; and among Notohippidae, which appear here for 

 the last time, Argyrohippus, which is particularly interesting from the thickness 

 of the enamel of its molar teeth. The Entelonychia already begin to be poorer 

 in forms, so that the important genus Colpodon occurring here appears all the 

 more abundant. Typotheria, Litopterna and A strapotheriidae, as well as Dasy- 

 iiridae, are unrepresented, which is especially remarkable ; but the circum- 

 stance that the Nesodontidae appear to be now typically developed should not 

 be overlooked. In age this fauna seems to correspond with that of the marine 

 Patagonian formation. 



The fauna of the Santa Cruz beds which now follows is the richest in 

 species and the best studied of the whole of the South American Tertiary. 

 Ameghino records several primates, among which only the genus Homunculus 

 certainly belongs to this order. The Nesodontidae — Nesodon and Adinotherium 

 — are remarkable for their great abundance ; among the Typotheria, the genera 

 Protypotherium, Interatherium and Hegetotherium, and the small Pachyruchos are 

 not much behind these in the number of individuals. The Entelonychia, of 

 which Homalodontotherium is the sole survivor, now become very rare, while 

 the A strapotheriidae are approaching extinction ; but among the Litopterna the 

 Proter other iidae attain their maximum development, while the Macraucheniidae 

 are distinguished from their forerunners by a considerable increase in bodily 

 size. The rodents become unusually numerous, both the Caviinae, Chin- 

 chillinae, and of Ododontidae, the Loncherinae and Capromyinae ; but of the 

 Hystricoidea, only the Acaremyinae exhibit a noteworthy development. Among 

 the edentates the Megalonychinae are distinguished by their special abundance, 

 while the Megatheriinae begin slowly to increase. The same may be said of the 

 Glyptodontia, while the Dasypoda remain almost unchanged both in their organ- 

 isation and in their abundance. Special mention should be made of the genus 

 Stegotherium, which, from its long snout and reduced dentition, may lead to 

 Myrmecophaga, as well as of the presence of an insectivore, Necrolestes, which 

 reminds us of the African Chrysochloridae. The marsupials now reach the 

 highest stage of their development in South America, not only the Basyuridae, 

 represented by the fine genera, Borhyaena and Prothylacinus, and the smaller 

 Cladosidis and Amphiproviverra, but also the Caenolestidae — Palaeothentes, 

 Abderites, and Garzonia, of which nevertheless only the last genus has left 

 descendants. Lastly, the presence of a didelphyid, Microhiotherium, is also 

 noteworthy. 



The faunas between the Upper Miocene and the Pleistocene in South 

 America are distinctly behind the earlier and later faunas, both in their 

 variety and in their investigation. The Parani stage is of marine origin, and 

 therefore among mammals contains only Cetacea, Pinnipedia, and Sirenia, but of 

 these a toothed whale, Prosqualodon, is of great interest. The land mammal 

 fauna is characterised by the rarer occurrence of the Litopterna and Typotheria, 

 VOL. Ill X 2 



