AESINOITHEEIUM. 



11 



behind the posterior border of the alveolus of the last molar. The facial 

 surface of the bone slopes inwards, especially in its anterior portion (see PI. II. 

 fig. 1 a). The suture between the maxilla and premaxilla crosses the alveolar 

 border immediately behind the last incisor. On the palate it runs backwards 

 for a short distance, behind which the anterior edge of the maxilla becomes free 

 and forms the lateral and hinder borders of the large median anterior palatine 

 vacuity (PL II. fig. 1). The anterior portion of the palatal surface formed 

 by the maxilla is very highly arched, so much so, in fact, that the sides of the 

 palate are nearly parallel with the facial surface of the bone (see text-fig. 3), the 



Text-fig. 3. 



i-fimx. 



mx.fimx.s. 



^jim.i. 



Skull of Arsinoitherium zitteli : diagrammatic section through snout at the level of the third premolar, 

 centre about which the teeth move in growth ; mes.g., mesethmoid groove ; mx., maxilla ; 



c. 

 m, 



x.pmx.s., maxillo-premaxillary suture ; pm. 3, third premolar ; pmx., premaxilla. | nat. size. 



distance between them being practically the width of the alveolar surface. This 

 peculiarity seems to be mainly due to the deepening of the alveolar region of 

 the bone in order to carry the high-crowned and long-growing cheek-teeth. 

 The diagram given in text-fig. 3 is a vertical section of the snout taken at the 

 level of the third premolar, showing that the teeth and the deepened portion of 

 the maxilla form a continuous curve which is almost a part of a circle Avith the 

 centre at the point c. The growth of the teeth alon^' this curve accounts, in a great 



c2 



