302 Bulletin American Museum of Natural History. [Vol. XXXIV, 
of individual differentiation in size and coloration renders difficult the 
satisfactory discrimination of local forms. True hoffmanni is found in the 
mountainous parts of central Costa Rica, and has a barely recognizable 
lowland form in Costa Rica and Chiriqui. South of Chiriqui there is a 
hiatus in its range, it next appearing in the Western Andes of Colombia, 
and it occurs thence throughout all the western and central Andean range to 
southern Ecuador, with a lowland form along the coast of Ecuador. The 
mountain form of the Western Andes is not satisfactorily distinguishable 
from specimens from 'the Irazti region of Costa Rica. In the northern part 
of the Central and in the Eastern Andes in Colombia, true hoffmanni breaks 
up into several distinguishable local forms, some of which approach, and 
may intergrade with, griseogena of the Sierra de Merida and Sierra de Mer 
of Venezuela, which in turn is not greatly different from chapmani of the 
Paria Peninsula and the Island of Trinidad. There is a slightly differen- — 
tiated form of chapman in Tobago Island, and another strongly specialized 
form in Margarita Island. This latter differs so widely in coloration from 
any of the mainland forms as to indicate its long isolation. 
The hoffmanni group is thus a group of wide dispersion and of very 
numerous forms, and perhaps may be regarded as one of the oldest groups 
of South American squirrels. Within its range are included the range of 
the genus Microsciurus and part of the range of the genus Leptosciurus. 
| The gerrardi group of the genus Mesosciurus (subgenus Histriosciurus) 
consists of a large number of forms of much larger size than the squirrels 
of the hoffmannt group, and of much more restricted distribution. They 
differ from the latter not only in conspicuously larger size, but in markedly 
different coloration and pelage. They are as remarkable for inconstancy 
of characters, particularly in coloration, as are the members of the hoffmanna 
group for constancy to a general type. The range of the gerrardi group is 
the western slope and adjoining lowlands of the Western Andes, from sea 
level to about 5000 feet, from northeastern Ecuador to Panama and the 
lowlands of the Caribbean drainage of Colombia, east to a little beyond 
the Venezuelan border. Not only do the extreme phases of the gerrardi 
group differ widely, especially in coloration, but specimens, with the ex- 
ception of two Panama forms, of the same form from the same locality 
vary so much that hardly two can be found that are closely similar. It is 
obviously a highly plastic group, of probably comparatively recent evolu- 
tion, and confined mainly to the humid tropical zone. So far as known, 
no two forms of, respectively, the hof'manni and the gerrardi groups occupy 
the same areas, although the boundaries of the ranges of the two groups 
must practically adjoin each other along the western slope of the Andes 
for nearly a thousand miles. The only other group of squirrels that shares 
the habitat of the gerrardi group is the genus Microsciurus. | 
