388 Bulletin American Museutn of N^atiiral Histo7y. [Vol. XIX, 



simultaneously with increasing stature and longer limbs, 

 partly to the necessity of reaching outside the lengthening 

 tusks, partly to the intrinsic advantages of a prehensile organ 

 of such manifold possibilities. On the one hand, we may 

 suppose, the enlargement of the trunk intensified the changes 

 due to the shifting and enlargement of the tusks, and on the 

 other hand inaugurated many of the peculiar conditions de- 

 scribed below. 



The backward shifting of the weighty tusks and trunk, 

 compensating their adverse leverage, lessened the antero- 

 posterior space available for the grinding series, while the 

 work put upon the individual grinders increased with larger 

 bodies and longer lives. This may explain in part why the 

 simple grinders of Dinotherhim, with their two, or at most 

 three, low, widely separated ridges, were constrained to 

 evolve into the wonderfully specialized and effective grinders 

 of the Mammoth, with ridges in great number, exceedingly 

 high, and closely appressed, and in which there is a so-called 

 horizontal succession in use, the unworn teeth being pushed 

 into place from behind. The weighty, rapidly heightening 

 molars, together with their immense, backward and upwardly 

 growing alveolar pouch, might thus be regarded as a third 

 factor in the evolution and individual growth history of the 

 skull. It is plain, however, that this factor was a minor one 

 and that the characteristic features of the skull were attained, 

 in great part, before the teeth had become so highly special- 

 ized; for the typical Mastodon americanus , with its great size, 

 and great tusk-and-trunk development, shows nearly the same 

 degree of backward and downward extension of the posterior 

 nares and hard palate as in Elephas, yet retains comparat- 

 ively primitive low-crowned grinders, and the considerable 

 space between the posterior border of the last molar and the 

 posterior lateral limits of the hard palate in this genus was 

 not nearly so much utilized for storing incoming grinders as 

 it is in Elephas. 



Principally then, to the enlargement and backward shifting 

 of the trunk and tusks and the consequent fore-and-aft com- 

 pression of the whole skull, combined with the progressively 



