AliSINOITIIEEIUM. 
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 (PI. 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. 
Skull of Arsinoitherium zitteli : diagrammatic section through snout at the level of the third premolar. 
c., centre about which the teeth move in growth ; mes.g., mesethmoid groove ; mx., maxilla ; 
mx.pmx.s., maxillo-premaxillary suture ; ynn. 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 with the 
centre at the point c. The growth of the teeth along this curve accounts, in a great 
c2 
