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. 13., “ Princeton Expedition,” Yol. V., pp. 384—386 and 
p. 413. 
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