394 Bulletin American Museum of Natural History. [Vol. XIX. 



side, at a point opposite the malars, where the skull is most 

 constricted laterally, these buttresses branch off externally 

 into symmetrically opposed arches in different planes, which 

 pass forward, backward, downward, or upward. The wedge- 

 shaped basisphenoid, situated between the inner ends of the 

 buttresses, is the veritable keystone of this converging system. 

 The skull as a whole is thus highly adapted to resist the 

 severe strains put upon it. The occiput, both in ontogeny 

 and phylogeny, flattens out and rotates backward, spreading 

 both vertically and laterally, until at last it forms, as it were, 

 a great, functionally solid bed-plate, receiving the thrusts of 

 the opposite inverted arches into which the skull has been 

 resolved. Each pair of these symmetrically disposed arches, 

 which also connect with the system culminating in the basi- 

 sphenoid, reacts, of course, against some component of the 

 force transmitted either to or from the tusks, trunk, and 

 grinders, or when the forehead is used in pushing. The in- 

 numerable toughly constructed air cells of the diploe give 

 immense strength, lightness and especially resiliency. This 

 desideratum may also be the reason for the very loose articu- 

 lation of the malar with the zygomatic process of the squa- 

 mosal, which would also permit the facial portion of the skull 

 to bend back slightly, under pressure, toward the cranial 

 portion. 



