THE SKULL 



important influence in modelling the external shape of the cetacean head. 

 That backward force offered by the water as the animal moved forward 

 has been requisite to the extreme condition of the telescoping of the 

 odontocete rostral elements, but it has been only a secondary factor, the 

 primary one consisting of muscular influence accompanying the extreme 

 recession of the odontocete type of narial musculature. That the moder- 

 ate forward tilt of the odontocete occipital plane was caused by muscular 

 conditions induced solely by the elevated and static position of the head 

 unaccompanied by the need for cephalic agility, and that as far as the 

 evidence points this region has been totally unaffected by backward pres- 

 sure of the water. That the elimination or lateral displacement and re- 

 duction of the central cranial elements of the Odontoceti have been 

 caused solely by backward crowding of the face and forward crowding of 

 the occiput, and hence by forces that were exclusively or chiefly muscu- 

 lar. That the telescoping and more excessive forward tilting exhibited 

 by the occipital region of mysticeti is due solely to muscle migration 

 finally caused by the static position of the head with need for the mini- 

 mum of cephalic movement. 



[137] 



