50 MOSTLY MAMMALS 
A somewhat different type of arrangement obtains in 
the hand of the South American spider-monkeys (fig. B), in 
which the thumb is wanting. In this group, although whorl- 
like patterns are observable in the interdigital eminences, yet 
they are much smaller and less distinct than in ordinary 
monkeys ; the same being the case with the ulnar eminence. 
The radial pattern at the inner side of the thumb is, 
however, practically wanting, owing doubtless to the absence 
of that digit. It will further be noticed from an examination 
of the figure that elsewhere on the palm, not even excepting 
the fingers, the general arrangement of the ridges is longi- 
tudinal. Since the hands of the spider-monkeys are, as 
already mentioned, largely used in a hook-like manner 
during the arboreal evolutions of these active creatures, it 
would seem at first sight that the arrangement of the ridges 
precisely controverts what has been said above as to their 
being parallel with the long axis of the object grasped. 
But the palms of even these monkeys, as is indicated by 
the numerous creases, are evidently much folded laterally ; 
and it must also be borne in mind that an equally important 
function of the hand is the plucking and holding of spherical 
or sub-spherical fruits. And for such a combination of 
functions the mode of arrangement of the ridges is doubtless 
the one most suitable. If the ridges were transverse, the 
fruit would very probably have a tendency to slip out of 
the hand on one side or the other; but this is clearly 
prevented by the longitudinal arrangement. 
The above are the chief modifications displayed by the 
palm-prints of monkeys; and it may be added that a very 
similar general plan of arrangement of the papillary ridges 
and grooves obtains on the sole of the foot of these animals, 
subject, however, to such modification as is necessary for 
the different function of the foot as compared with the 
