_Chrysothrix_, Kaup., Thierreich., i., p. 51 (1835).
{153}The Saimiris, or Squirrel-Monkeys, are very beautiful and active
little animals, characterised by their soft, close, and erect fur, and
especially in having the head produced posteriorly. The face is relatively
small and has a high facial angle. The eyes are large, directed forwards,
and set very close together. The ears are large; and the nose has a very
broad partition between its nostrils. The tail is long, round, and covered
with short hair; but tufted at the extremity and non-prehensile.
As regards the skeleton, the skull is elongated, and the arched cranial
portion prolonged backwards, the length of the base of the skull being
shorter than the cerebral cavity. The facial portion of the skull is
relatively smaller and the cranial larger even than in Man; this character
being, however, common to all the smaller representatives of particular
groups, and "obviously necessary to provide the requisite amount of
brain-space." (_Mivart._) The angle of the lower jaw is narrow behind. The
bony partition between the nostrils is very thin and membranaceous; and
that between the large orbits is also thin and imperfect. The lower incisor
teeth are vertical, and in regular series with the canines, and the latter
are well developed. No Primate has the teeth placed in one uninterrupted
series except Man; but there is always a small interval between each upper
canine and the adjacent incisor, and between each lower canine and the
adjacent pre-molar.
The skeleton of the hand is one-fifth of the length of the spinal column.
The wrist-bones are nine in number, the central--_os centrale_--being
present as in the majority of Monkeys. In _Chrysothrix_ and in
_Nyctipithecus_ also, the thumb is proportionately shorter than in any
other genus, except among the Spider-Monkeys (_Ateles_), and the Old World
{154}Guerezas (_Colobus_). In the length of their foot the members of this
genus approximate to the proportion existing in Man; and its length,
compared with that of the hand, is greater in _Chrysothrix_ than in any
other group of Monkeys.
The cerebral hemispheres project beyond the hind brain (_cerebellum_) to a
greater relative extent than in any other mammal, namely, to one-fifth of
their total length. (_Huxley._) The external surface of the cerebral
hemispheres is almost as little convoluted as in the Marmosets and
Tamarins, in which it is almost quite smooth, yet on the inner faces of the
hemispheres the more important grooves (_sulci_) are present. The opening
for the passage of the spinal cord lies nearly in the middle of the base of
the skull, whereas in other genera it is situated closer to the hinder
region.
The Squirrel-Monkeys are entirely arboreal, and found in most of the
tropical regions from Costa Rica to Brazil and Bolivia, being among the
commonest of the Anthropoids of the American forests. They are diurnal, and
feed chiefly on insects; but they will also often attack and devour small
birds.