xviii INTEODUCTION. 



condition of the specimens examined is not favourable to its preservation. The canine 

 and the first premolars are lost; the other premolars are evidently undergoing 

 reduction, but at the same time the last has become bilophodont. The molars are 

 all trilophodont, each transverse crest consisting of at least two distinct tubercles ; 

 there is also a tendency to the addition of aiiother posterior ridge from the cingulum. 

 One notable point about the molar series is the sudden enlargement of the second 

 and third true molars, compared Avith the teeth in front. This is accompanied by 

 the considerable development of cellular tissue in the posterior part of the maxilla 

 and the neighbouring bones, and appears to be the first indication of the great 

 increase in the size of the posterior molars found in later types. This increase in 

 the size of the maxilla, vehich is probably correlated with the general tendency of the 

 bones of the posterior part of the skull to develop diploe, seems almost as if it were 

 a preparation for the subsequent changes in the teeth, but, on the other hand, it is 

 possible that the increase in their size may be in part a consequence of possessing 

 more room in which to develop. 



As already noted, the mandible is greatly elongated in the symphysial region, and 

 its ascending ramus slopes backwards, both modifications tending to increase the 

 distance the animal could reach with its lower incisors. Of these there is a single 

 pair (the second) projecting forwards from the symphysis : the canines and first and 

 second premolars are lost; the other premolars and molars are generally similar to 

 those of the upper jaw. In the vertebral column the axis still has a peg-like 

 odontoid process and a comparatively long centrum, though the centra of the posterior 

 cervicals are much shortened. The limb-bones, so far as known, are closely similar, 

 except in size, to those of Elephas. 



In general appearance Palceomastoion must have resembled a small rather long- 

 necked Elephant, the most notable difference being that the trunk, instead of being 

 freely flexible, was supported by, and formed the upper covering of, the elongated 

 mandibular symphysis ; its extremity, however, may have been free and to some degree 

 prehensile. 



The further history of this group can only be briefly summarised here (see Phil. 

 Trans. 19G B, 1903, p. 99). The next form, Tetrahelodon angustidens from the European 

 Lower Miocene, has the symphysis still more elongated and the narial opening shifted 

 further back. At the same time the molars, or at Irast the posterior ones, are greatly 

 increased in size and possess more transverse ridges. There are three milk-molars 

 in, both jaws, and the posterior two are succeeded in the normal way by premolars; 



