HISTOEY OF THE PROBOSCIDEA 439 



striking changes. At first (jMoeritherium) they were small, 

 very low-crowned and of simple pig-like or quadritubercular 

 pattern, making two interrupted cross-crests ; all were in use 

 simultaneously and the succession of milk-teeth and premolars 

 was by vertical replacement, as in normal mammals generally. 

 In ^Palceomastodon there were three pairs of tubercles on the 

 molars and in 'fGomphotheriwm these coalesced into ridges, but 

 in all the fmastodons there was more or less distinctness of the 

 conical tubercles. In one or more phyla the three-ridged plan 

 persisted for a long time, one such phylum terminating in the 

 Pleistocene "fMastodon americanus. In the other series the 

 number of ridges increased, first to four, then to five, six and 

 more {'fStegodon) ; the crowns of the teeth became much 

 larger and higher, and the ridges, as their number increased, 

 became much thinner, and the valleys between them were filled 

 with cement, and finally, in the true elephants, with their 

 fully hypsodont, many-crested teeth, were thickly covered all 

 over with cement. The vertical succession of milk-teeth and 

 premolars was retained in '\Gomphotherium, at least in some 

 species, but the large molars, which could not find room to be 

 exposed while the premolars were in place, came in successively 

 from behind. This horizontal mode of succession is the only 

 one to be seen in the true elephants, in which but one tooth, 

 or parts of two, on each side of each jaw are in simultaneous 

 use and the premolars have entirely disappeared, but the milk- 

 teeth are retained. 



The changes in the skull, which amounted to a recon- 

 struction, were very largely conditioned by the great increase 

 in the length and consequent weight of the tusks, in the size 

 of the grinding teeth and the development of the proboscis. In 

 the earliest known type {]M(Eritherium) the skull had little 

 about it that would, at first sight, suggest proboscidean affin- 

 ities ; it was long and narrow, with sagittal crest and occiput 

 of normal type, very long cranial and very short facial region. 

 The nasal opening was directed forward and the nasal canal 



