8 ON THE MANDIBLE OF ZYGOMATUEUS. 



premolar have suffered damage. On the left side the last three 

 molars are well preserved ; the foremost and the premolars have 

 been ruined by exposure to the accidents of burial and removal. 

 Both ascending limbs are broken away — that of the left side 

 with a clean fracture which yields important information. 



The fracture traverses the base of the coronoid process a 

 little above its junction with the horizontal ramus, ascends to a 

 little above the level of the angle of the post-dental platform and 

 then passes nearly horizontally through the ascending ramus. 

 The coronoid process rises, not gradually from a narrow, rapidly 

 attenuating, and obliquely set base, but abruptly from a broad 

 base retaining its thickness to a great degree as it recedes and 

 with a longitudinal axis parallel with the Ime of the teeth. 

 Opposite to the middle of the post-dental platform it has a 

 thicknesss of 21 mm,, almost twice that of the bone in 

 Nototherium in which it is 12 mm. ; its outer margin is here 

 51 mm. distant from the angle of the platform against 44-5 in 

 Nototherium. Further back, we see evidence of a similar 

 inordinate thickening in the ascending ramus ; between the two 

 fossas the fracture attains a breadth of 37 mm., whereas at the 

 same point in Nototherium the bone is but 15 mm. in thickness. 

 Furthermore the posterior surface of the ascending ramus has a 

 breadth of 55 mm., while that of Nototherium between the same 

 points is only 40-5. The extraordinary strength of the 

 posterior portion of the mandible indicated by these measure, 

 ments, is in accord with the massiveness of the whole cranium . 

 and was necessitated by the great volume of the masticating 

 muscle, itself demonstrated by the outwardly bulging and 

 ponderous zygoma. But incrassation was not the only effect 

 which the enormous muscle had on this region of the jaw ; the 

 thickness of that portion of it which was attached to the inner 

 surface of the ascending limbs of the mandible forced these 

 asunder to a proportionate extent, while the tooth-bearing 

 porti'm, having to remain in correspondence with the less 

 increased width of the upper jaw, remained less expanded ; the 

 consequence is that the angles of divergence of the facial and 

 articular parts of the mandible are different. The ascending 

 ramus which in NotDtherium, is in fore and aft direction almost 



