242 THE TRANSFORMATIONS OF THE ANIMAL WORLD 



It is equally possible to point out in the Tertiary 

 Mammals, especially in the cranial characteristics, 

 primitive stages which may be set against other 

 stages of more advanced specialization, or senile 

 stages. These primitive or archaic characteristics, 

 which are found on parallel lines in groups quite 

 independent of each other, are, among others, as 

 follows : (1) the bones of the cranium are distinct 

 or joined only by sutures ; (2) the longitudinal 

 profile of the head is depressed and rectilinear, 

 rising only a little or not at all towards the occiput ; 

 (3) the snout is long, and the well-developed nasal 

 bones are articulated to the pre-maxillaries ; (4) 

 the orbit is opened out backwards, and communi- 

 cates with the temporal fossa ; the frontal and 

 parietal regions are smooth, without projecting 

 crests, antlers, or horns ; (6) the glenoid cavity 

 of the articulation of the mandible is of small 

 depth, and allows of movements in every direction ; 

 (7) the two branches of the mandibles are joined by 

 ligaments, instead of being welded together. The 

 senile stages naturally answer to opposite charac- 

 teristics, such as the welding together of the skull 

 bones ; the raising of the profile of the head to- 

 wards the rear ; the shortening of the nasal bones ; 

 the closing of the orbit ; the presence on the cranium 

 of protuberant ridges, branches, horns, or antlers ; 

 the limiting of the movements of the mandible ; 

 and the welding together of the branches of the 

 mandible. It should not be forgotten that, as 

 with the Nautilidae, these stages of development do 

 not manifest themselves in all branches at the 

 same period of their geological course, the rapidity 



