THE CAPUCEIINS, HOWLERS, AND SPIDER-MONKEYS. I51 



Monkeys there are no cheek-pouches, nor ischial callosities. Ex- 

 cept in the Spider-Monkeys the hind-limbs are longer than the 

 fore- ; " while the thumb, even where it is best developed, is 

 capable of but a partial opposition to the other fingers, bending 

 almost in the same plane with the latter, so as to be more like 

 a fifth finger." {Mivaj-t.) Nevertheless, all its muscles, except 

 the long flexor, are present. The great-toe is large and can be 

 moved from and to the side of the other digits, but is not 

 opposable to them. 



The skull is smooth and has no muscular crests ; the ex- 

 ternal bony tube to the ear is not ossified. The two extremes 

 in its form are presented by the Howling Monkeys {Alouatta) 

 and the Squirrel-Monkeys {Chrysothrix)^ as pointed out by 

 Professor Huxley in his " Anatomy of Vertebrated Animals " : 

 *' In the former the face is very large and prominent, with a low 

 facial angle. The roof of the brain-case is depressed; the 

 plane of the occipital foramen [for the passage of the spinal 

 cord] is almost perpendicular " to the axis of the base of the 

 skull. In Chrysothrix^ on the contrary, the face is relatively 

 small, with a high facial angle ; the brain-case is moderately 

 arched ; " and the plane of the occipital foramen is horizontal. 



The dentition of the CebidcB is very characteristic of the 

 family. The dental formula is If, C|, P?, M| and the teeth 

 thirty-six in all — a larger number than is found in any of the 

 Old World forms, or in the species of the last family (the 

 Hapalidce) ; for they possess an extra pre-molar tooth above 

 and below on each side. Their molar teeth are four-cusped ; 

 and in the upper molars of the Spider-Monkeys {Atehs) and 

 of the Howlers {Alouatta) there is an oblique cusp, such as is 

 found in the molars of the Lemuroidea, joining the hind inner 

 to the front outer cusp. Among the Cebidce the brain varies 



