434 LAND MAMMALS IN THE WESTERN HEMISPHERE 



It is possible to take another and very long step back from 

 ^PalcBomastodon, so long, indeed, as to make it apparent that 

 one or more links in the chain are still missing. The genus 

 \Mceritherium is found together with ^Palceomastodon in the 

 lower Oligocene, but also occurs separately in the upper Eocene. 

 It seems likely that it is a persistent middle Eocene type and 

 that the known species of it were somewhat aside from the 

 main line of descent, but that it very closely represents, never- 

 theless, a very early stage in the elephant genealogy. These 

 known species were quite small animals, about the size of a 

 tapir, and therefore not much less than the smaller members of 

 \Pal(Bomastodon. The dental formula of ^Mosritherium was : 

 i i> c T) P f ) TO f ) X 2 = 36. The first or median upper incisor 

 was a relatively small and simple tooth, but the second was 

 quite a large, downwardly directed tusk, which was much 

 smaller and less curved than in f Palceomastodon, and was not 

 capable of indefinite growth. The third incisor and the canine 

 were small, spike-like teeth of no functional importance, but 

 their presence is significant as approximating the primitive, 

 unreduced dentition of the ungulates. The lower incisors 

 were nearly procumbent, with a slight upward inclination; 

 the first one was long and the second a thick, enamel-covered 

 tusk, with a chisel-like edge, which was produced by wear. 

 The premolars were smaller and simpler than the molars, which 

 were quadritubercular, the four conical cusps arranged so as 

 to form two transverse crests, giving a pattern like that of the 

 early pigs and peccaries and of precisely the kind that might 

 have been predicted from the teeth of ^Palceomastodon. 



The skull had an utterly different appearance from that 

 of ^Palceomastodon, the difference being much greater than 

 between the latter and the Miocene \Gomphotherium. It was 

 long and narrow, and, except for the very prominent zygomatic 

 arches, of nearly uniform, tubular shape, the brain-case being 

 of small capacity, though, as compared with other Eocene 

 mammals, the brain was proportionately large. "It is possible 



