Marsh Collection, Peabody Museum. 171 



potomy) divisions of the suboccipital nerve. This difference 

 in the arrangement of the canal is constant and well marked, 

 and serves as a reliable guide for distinguishing certain of the 

 groups. The latter condition is also found in the Insectivora, 

 and may be therefore looked upon as the more primitive of 

 the two. 



Finally, I may mention the absence of a separate opening 

 (foramen spinostunj, for the entrance of the middle menin- 

 geal artery into the cranial cavity, in all Primates except man. 



Characters of the Prehensile Extremities. 



The second set or group of characters of fundamental 

 importance, by means of which the Primates are distinguished 

 from all other orders of the Mammalia, relates to the modifi- 

 cation of the hands and feet into more or less perfect 

 prehensile or grasping organs, and in this respect they stand 

 sharply apart from all other divisions of Eutherian mammals. 

 Of the causes which led to this modification, very little is 

 known, but there can be hardly any doubt that it was one of 

 the primary distinguishing features of their remote Metathe- 

 rian ancestors far back in the Mesozoic, and that its appear- 

 ance constituted one of the first steps which led to their 

 subsequent differentiation into such a distinct group. The 

 particular type of extremity from which the prehensile modi- 

 fication arose was undoubtedly that of the plain plantigrade 

 condition ; and the assumption of an arboreal manner of life, 

 we may readily believe, was the all-important determining 

 factor in its evolution. 



Exclusive of man, the Primates are, without exception, the 

 most preeminently arboreal in habit of all the Mammalia. 

 The arrangement of the thumb and great toe in opposition to 

 the other digits, thus making it possible to take a firm hold or 

 grasp upon the branches of a tree, is one of vast superiority to 

 that in which the digits depend for their effectiveness upon 

 sharp claws. 



In the latter case, the claws serve as so many hooks by 

 means of which the weight of the body is sustained in the 

 act of climbing, whereas in the prehensile hand or foot of the 

 Primates the hold is rendered effective by the opposability of 

 certain of the digits to each other. The advantage of the 

 grasping extremity over that which is solely dependent upon 

 the hooked claws is seen in the ability of the animal possessing 

 the former to traverse the forest without descending to the 

 ground. Where the large branches of the adjoining trees 

 interlace, the clawed animal experiences little difficulty- in 

 passing from one to the other, but if the trees are scattering, so 

 that only the small branches touch, his progress is barred. 

 To the animal with prehensile extremities, however, the case 



