PH. C. W. ANDREWS ON THE EVOLUTION OE THE PKOIiONCIDEA. 



to I lie cxt raordinaril v complex 1 vpe of tooth found in H/ep/ias. Tims in Pnhr.omastodon 

 (lie molars are trilophodout, and the same is true of the first and second molars of 

 Tetrabdodon, in which, however, the last molar is complicated hy the addition of 

 further transverse crests. In (lie Stegodonts of the Siwalik Hills a further increase 

 in the number and height of the crests takes place, and the whole crown of the tooth 

 is more or less covered with a thick coat of cement. Still later the transverse crests 

 become highly compressed laminae united by cement, and in Elephas maximus and 

 E. primigciiius there may be as many as twenty-seven of them. 



In the lower jaw the median and outer pairs of lower incisors (1. 1 and 3) were 

 soon lost, but the second pair (I. 2) remained functional, and being prolonged in the 

 direction of the mandibular symphysis helped to lengthen the animal's reach. After 

 the symphysis became shortened, the lower incisors were in some cases (e.g., Mastodon 

 americanus) retained, but the irregularity of their occurrence in known specimens 

 shows that they were probably functionless. In Elephas no trace of the lower incisors 

 remains. 



The changes undergone by the lower molars are similar to those of the upper 

 molars. 



