178 NEBRASKA GEOLOGICAL SURVEY 



Otherwise there would not have been the extent of surface neces- 

 sary for the attachment of muscles and ligaments to carry the 

 ever-increasing load. This expanse of surface was brought about 

 by the inflation of air cells in the cant ellous tissue. This structure 

 not only surrounds the brain, but extends into the maxillae, zygo- 

 matic arch, and other parts of the skull. 



At first the proboscis was small, but little more than a pre- 

 hensile lip, much like that of the horse, and used in much the 

 same way, to guide food into the mouth. In the later mastodons, 

 mammoths, and modern elephants, perfection of specialization 

 was attained by this remarkable organ. This too added to the 

 weight of the skull. 



THE MANDIBLE 



The lengthening and subsequent shortening of the mandibular 

 tusks was paralleled by the lengthening and subsequent shortening 

 of the mandible itself. The remotest known ancestor of the group 

 had mandibles of a typical mammalian form, which in successors 

 became increasingly atypical. As the mandibular tusks developed, 

 mastodons became longirostral to the last degree. The symphysial 

 prolongation, which was extreme in Mastodon angustidens, is re- 

 duced in the mammoths and present elephants to a mere process 

 on the front of the jaw. 



THE TEETH 



The ancestral proboscidean had a number of teeth, after the 

 manner of typical mammals, and these were erupted in the usual 

 way, that is by new teeth pushing upward and gradually displacing 

 the old. These are called teeth of vertical succession. But inci- 

 dent to progressive change, and extreme specialization, the teeth 

 of mastodons and mammoths became too large for the jaw to 

 accommodate many at a time, and finally the dentition was reduced, 

 in fully matured adults, to one large specialized tooth in each 

 jaw. These teeth were erupted in a singular manner, that is, by 

 the one behind crowding forward, and pushing out the one in 

 front. This is known as the horizontal succession of teeth. Ac- 



