168 Bulletin American Museum of Natural History.  [Vol. XXXIV, 
sciurus, have each a fairly constant or uniform style of coloration, while in 
another group of the last-named genus it varies not only with each sub- 
species, but in some of the subspecies the range of individual variation is so 
excessive that it is difficult to find two specimens from the same locality 
that are wholly alike, with extremes that are exceedingly diverse. Varia- 
bility in coloration is as much a feature of this group as constancy to a single 
type of coloration is a feature of certain other groups. In general, South 
American squirrels, large or small, have red bellies, or underparts that are 
washed or suffused with buff, ochraceous, or deep ferruginous; a few;.only 
have pure white bellies. There is a tendency to a black dorsal area in many 
species of the Andean region — restricted and not strongly developed in 
some of the forms of Leptosciurus and Microsciurus, but strongly developed 
in some of the subspecies of the hoffmanni group, and in all of the subspecies 
of the Mesosciurus gerardi group, in some of which the median half or more 
of the dorsal area is deep black, with the limbs and flanks red. In most of 
the smaller species of South America the upperparts are gray, suffused more 
or less strongly with fulvous or rufous; in the larger species, brown strongly 
washed or suffused with ferruginous. 
Melanism, in the usual sense, is rare in South American squirrels, the 
only known instance being Sciwrus flammifer Thomas of the Orinoco Valley, 
in which about half of the known specimens are strongly melanistic. A few 
melanistic specimens of other species have been recorded, being noteworthy 
on account of the rarity of such occurrences in South America. On the 
other hand, melanism is a widespread local condition among the gray and 
fox squirrels of eastern United States, and a common condition on a large 
scale in some of the squirrels of southern Mexico and Central America." 
In a general way peculiarities of coloration among South American squir- 
rels may be considered as indicative of group affinities. 
Skull and teeth. 
In attempting to discover tangible differences in the form of the skull and 
in the character of the teeth in American squirrels, with a view to their use 
as the basis of generic or subgeneric divisions, some surprises have been met 
with in respect to the variability of such features in specimens of the same 
1 In the humid tropical lowlands of western Colombia, western Ecuador and adjoining 
portions of Peru, irregular small white spots, consisting of tufts of elongated white hairs, are 
of more or less frequent occurrence in all the squirrels inhabiting this region, including species 
of Microscturus, Mesosciurus, and Simosciurus. They occur most frequently on the limbs 
and dorsal surface, but appear also on the ventral surface. Their contrast in color with the 
surrounding pelage renders them conspicuous marks, but they are obviously pathological, 
caused probably by bites of insects, and have no taxonomic significance. 
