444 



Drs. D. Hepburn and D. Waterston. 



[Jan. 23, 



manner, and these teeth remain in wear simultaneously with the true 

 molars ; but in later forms no vertical succession takes place, and as 

 the milk-molars are worn they are shed, being replaced from behind by 

 the forward movement of the molars. Of these also the anterior may 

 be shed, until at length in old individuals of the later types the last 

 molar is alone functional. The gradual increase in the complexity of 

 the proboscidean molars is one of their most striking characteristics. 

 All stages can be traced between the simple, brachyodont, bilophodont 

 (quadritubercular) molars of Moeritherium (Middle Eocene) to the extra- 

 ordinarily complex type of tooth found in Elephas. Thus in Palceo- 

 mastodon (Upper Eocene) the molars are trilopbodont, and the same is 

 true of the first and second molars of Tetrabelodon (Miocene), in which, 

 however, the last molar is complicated by the addition of further 

 transverse crests. In the Stegodonts of the Siwalik Hills (Pliocene) 

 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 com- 

 pressed laminse united by cement, and these are r as many as twenty- 

 . seven in number in the Pleistocene Elephas primigenius and the recent 

 E..indicus. 



The evolution of the lower molars corresponds with that of the 

 upper molars. Of the lower incisors the middle and outer pairs (Nos. 

 1 and 3) are soon lost, but the second pair remains functional for a 

 long geological period. When the symphysis becomes shortened, 

 these incisors are sometimes retained as vestiges (e.g., in Mastodon 

 americanus), but in the genus Elephas they have completely dis- 

 appeared. 



u A Comparative Study of the Grey and White Matter of the 

 Motor Cell Groups, and of the Spinal Accessory Nerve, in the 

 Spinal Cord of the Porpoise (Phoccena communis)" By David 

 Hepburn, M.D., and David Waterston, M.A., M.D. Com- 

 municated by Sir Wm. Turner, F.B.S. Pteceived January 23, 

 —Read March 12, 1903. 



(Abstract.) 



Recent advances in our knowledge of the arrangement of the motor 

 cells in the anterior cornua of the spinal cord of man have been made 

 almost entirely by the study of the changes produced in these cells 

 by the division or removal of limbs or parts of limbs in the human 

 subject, and very little has, as yet, been done to elucidate this subject 

 hy the comparative method of investigation. 



