FOSSIL RODENTS 403 



the same as in the recent ones. There is reason to believe, 

 says Professor Scott,* that several of the genera represent 

 the direct ancestors of existing forms. Viscacha was probably 

 derived from Prolagostomus, Dolichotis from Schistomys, 

 Erethizon and Coendu from Steiromys. As regards the latter, 

 it is interesting to note that it resembles the recent Erethizon 

 more than it does the modern South American genera 

 of the same family. It seems to me probable, therefore, 

 as I have already suggested (pp. 69 71), that the South 

 American and North American tree porcupines originated 

 from two distinct branches of ancestral Steiromys-like rodents, 

 thus favouring the view I advocated of a direct land connec- 

 tion between Patagonia and south-western North America. 

 That the genus Erethizon, to which all the North American 

 porcupines belong, should not be known from pre-Pleistocene 

 deposits presents, no doubt, a difficulty to the acceptance of 

 this theory, but its ancestors may have remained on the last 

 remnants of the land which once existed westward of North 

 and South America until compelled to leave that land in Plio- 

 cene times, when it finally subsided. 



The Santa Cruz fauna likewise reveals an affinity with the 

 fauna of Australia and Tasmania. The Patagonian marsupials 

 are referable to three families, remnants of which survive in 

 widely separated parts of the world. The Thylacinidae, now 

 confined to Tasmania, where the Tasmanian wolf represents 

 the family, formerly inhabited both Patagonia and Australia. 

 As we might expect, the Santa Cruz thylacines are of a more 

 primitive type than the Tasmanian wolf, but Professor Sinclair 

 expects that the common ancestor of these two will probably 

 be found among the marsupials occurring in still older Pata- 

 gonian deposits. The opossums (Didelphyidae), among which 

 Microbiotherium is the best known, are met with in several 

 genera in the Santa Cruz beds. The Cretaceous Proteo- 

 didelphys suggests, as I have already mentioned, that South 

 America must be looked upon as the original home of the 

 family whence some members passed into North America and 

 Europe. I have briefly alluded to the occurrence in Ecuador 



' 



* Scott, W. B., "Princeton Expedition," Vol. V., pp. 384386 and 

 p. 413. 



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