302 EECENTLY DISCOVEEED TERTIAEY VERTEBEATA OF EGYPT. 



this animal, which is as large as an elephant, the skull is practicalh^ 

 the same as that of the later Proboscidea ; the tusks are noAv A-ery large, 

 though they are still directed somewhat downward, and have a band 

 of enamel on their outer side. The milk molars, as in the earlier 

 forms, are still replaced by premolars ; but these are soon pushed for- 

 ward and shed through the great increase in size of the permanent 

 molars. Of these the first and second, though large, still have 

 crowns with only three transverse ridges; the third molar, on the 

 other hand, is still more enlarged, and its crown may be made up of 

 five or six transverse crests ; it is, in fact, so large that when it is fully 

 cut not only the premolars but also the first molars are displayed, 

 there being no room for them in the jaw. The anterior part of the 

 mandible, with the procumbent incisors, has now attained an extra- 

 ordinary length, projecting still farther beyond the skull than in 

 Palceomastodon : in fact, in this genus we have the culmination of the 

 specialization in this direction, and the long, straight snout must 

 have presented a remarkable appearance, the animal having resem- 

 bled an elephant in which the lower jaw was so elongated that it 

 could reach the gi'ound. and was coA'ered with the fleshy snout, the 

 end of which alone was free. So far as the Eg;\'ptian deposits are 

 concerned, this is the last of the Proboscideans found ; but it may be 

 permitted to give a short summary of the subsequent changes which 

 ended in the evolution of the modern genus EJeylias. During the 

 jMiocene the long mandibular symphysis — proljal^ly because it had 

 attained an unwieldly length — became rapidly shortened up, leaving 

 the upper lip and snout free, as the movable proboscis so character- 

 istic of the group. Tetrahelodon longirostns^ of the late ]\[iocene, 

 represents a stage in this process. In this animal the symphysis is 

 comparatively short, and although the two lower tusks attain a con- 

 sideraljle size, they certainly could not reach the ground. At the 

 same time the number of ridges in the molar teeth is increased to four 

 in the two anterior ones. In the Pliocene the mandibular symphysis 

 becomes still more shortened up, but in some species of Mastodon the 

 lower incisors still persist, though of small size and usually soon 

 shed. The number of transverse ridges in the molars increase and 

 become deeper, till in Stegodon (from the Pliocene of the Siwalik 

 Hills) the anterior molars rao.j have six or seven ridges, the last one 

 eight or nine. The valleys in these teeth are deepened, and may be 

 more or less filled with cement. At the same time, in most cases, the 

 milk molars are displaced by the development of the molars behind 

 them, before they can be replaced from below by the premolars. In 

 Elephas proper the elongated mandibular symphj^sis of the early 

 forms is represented by a small process forming the chin of the man- 

 dible and never bearing any trace of lower incisors. The molars 

 acquire a much greater number of transverse ridges and become 



