440 LAND MAMMALS IN THE WESTERN HEMISPHERE 



was relatively long and horizontal in direction, but the nasal 

 bones were already much shortened, indicating that the pro- 

 boscis was probably in an incipient stage. The symphysis 

 of the lower jaw was procumbent and somewhat elongated, but 

 to only a comparatively slight degree. 



While the skull of \M(jeritherium was not obviously pro- 

 boscidean, that of its successor, '\Palceomastodon, was unmis- 

 takably so, yet retained several primitive features, which were 

 lost in all of the subsequent genera, such as the sagittal crest, 

 the relatively low cranium and moderate thickening of the 

 cranial bones, the forward direction of the nasal opening, etc. ; 

 the symphysis of the lower jaw was very greatly prolonged. 



As the tusks enlarged and the proboscis grew longer, the 

 weight of the head and its appendages necessitated a largely 

 increased area of attachment for the neck-muscles, and this 

 was attained by a very great thickening of the cranial roof, 

 the occiput not increasing proportionately ; at the same time, 

 the thickened bones were honeycombed with sinuses, so as to 

 reduce their weight without sacrifice of strength. In those 

 species of the Miocene ^Gomphotherium which had large and 

 heavy tusks, this thickening was not very much less than in 

 the true elephants. The enlargement of the tusks had other 

 consequences, as, for example, in lengthening and broadening 

 the premaxillaries and, in the elephants, in their downward 

 bending, so as to shorten still further the facial region of the 

 skull. With the development of the proboscis, the nasal 

 bones were reduced to a minimum and the anterior nasal 

 opening was no longer directed forward, but obliquely upward, 

 while the nasal passage lost its horizontal direction and became 

 almost vertical. The lower jaw continued to elongate the 

 symphysis, reaching a maximum in certain species of ^Gom- 

 photherium ; but the reverse process of shortening this anterior 

 region of the jaw began with the reduction of the lower tusks, 

 and, when these had disappeared, nothing remained of the 

 immensely elongated symphysis, except the short spout of the 



