PHYLETIC BRANCHES AMONG THE VERTEBRATES 181 



resolves itself, it would seem, into a gradual and 

 very perceptible increase of size from the relatively 

 small form of the beginning, the D. cuvieri of the 

 sands of the Orleanais up to the enormous animal 

 of the Pontic strata of Roumania, named by 

 Stefanesen D. gigantissimum, and without doubt 

 the most formidable terrestrial mammal who lived 

 in geological times. On the contrary, the second 

 branch, that of the Mastodons with molars bristling 

 with conic mounds, offers some somewhat important 

 modifications of structure. Indeed, the Mastodon 

 angustidens of the lower and middle Miocene is an 

 animal relatively small and of low stature, and his 

 short limbs in no way recall the huge pillars of 

 the modern Elephant. His jaws are furnished with 

 four nearly straight tusks, two long ones in the upper 

 jaw and two rather shorter in the mandible, which 

 wear away by friction against the upper. In the 

 Mastodon longirostris of the upper Miocene the general 

 dimensions are much greater and the stature higher. 

 The upper tusks remain very long, but the lower 

 ones are much diminished in proportion, and hardly 

 attain 0- 50m. beyond the mandible. In the Mastodon 

 arvernensis of the Pliocene the height, the molars, 

 and the long upper tusks are almost identical with 

 those of the Mastodon longirostris, from which it 

 would be difficult to distinguish it if the lower 

 tusks had not entirely disappeared together with 

 the bony sockets in which they were fixed, so that 

 the symphysis of the mandible is short and bent 

 over downwards, instead of being plainly projected 

 forward, as was the case with the Miocene Mastodons. 

 In this case the functional modification goes as far 



