100 
JOURNAL OF MAMMALOGY 
Therefore, the two species may well occur together, for although both 
are undoubtedly arboreal, they may occupy different ecological niches, 
and have different food habits. It is interesting to note that P. albipes 
is found on the floor of these humid forests, its association seeming to 
be definitely riparian, and thus there are three distinct species of this 
genus inhabiting the same faunal area — a condition that is, so far as 
known, unique. 
The relationship of silvicolus is not clearly indicated. As mentioned, 
the external appearance places it with longicauduSj and it is probably 
closest to that form, but its skull does not resemble skulls of longicaudus 
to any greater degree than it does skulls of albipes, and the apparent 
similarity may be due solely to the influences of parallel habits acting 
upon two separate stocks. 
Pasadena, California. 
