THE SPIDER-MONKEYS. 229 
which, usually, a single minute nodular phalanx [finger-bone | 
is articulated, and is completely hidden beneath the integu- 
ment. Although thus rudimentary and functionless, all its 
characteristic muscles, except one (the long-flexor) are present. 
(Huxley.) The upper incisors are unequal, the interior being 
the larger. ‘There is a space (diastema) between the incisor 
and the canine teeth (as in all Az¢thropoidea, except Man) ; the 
canines are large and conical; the upper molars large, and 
their crowns four-cusped, with transverse ridges between the 
outer and inner front cusps and the outer and inner hind cusps, 
and also an oblique ridge crossing from the outer front cusp to 
the inner hind one. In the larynx of Aze/es there is a single 
median air-sac opening from the back of the windpipe, but there 
is no such extension of the resonating apparatus as is seen 
in the Howlers (A/ouwatta). In its brain AZeles exhibits in 
some respects a higher type than in even the Old World 
Apes. 
In regard to this group of Monkeys, the late Mr. H. W. 
Bates made the following interesting observations :—“ In the 
Coaitas the tail reaches its highest perfection as a prehensile 
organ ; and on this account it would perhaps be correct to 
consider the Coaitas as the extreme development of the 
American type of Apes. As far as we know from living and 
fossil species, the New World has progressed no further than 
the Coaita towards the production of a higher form of the 
Quadrumanous order. The tendency of Nature here has been, 
to all appearance, simply to perfect these organs, which adapt 
the species more and more completely to a purely arboreal 
life ; and no nearer approach has been made towards the more 
advanced forms of Anthropoid Apes, which are the products of 
the Old World solely. ‘The taii of the Coaita is endowed with 
