12 TEETIAEY VEETEBEATA OE THE EAYtM. 



measure at least, for the much greater height of the outer side of the crowQ than of 

 the inner side. The same diagram also shows in section the junction of the maxillae 

 with the premaxillse (tjcx.pmx.s.), and the probably mesethmoid groove (mes.g.) 

 which lies between the upper ends of the premaxillse and was no doubt occupied 

 in life by the lower edge of a vertical cartilaginous nasal septum. Another possible 

 explanation of the deeply concave palate is, that Arsinoitherium may have possessed 

 a tongue of peculiar form, probably extensible and prehensile ; for it is difficult to 

 understand how an animal of such great size could grasp sufficient food with so 

 extremely narrow an incisor region if it had not some additional means of prehension 

 in the form of a mobile upper lip or tongue. 



Further back the palatal surface of the maxillae becomes more flattened, and opposite 

 the second molar these bones unite with the palatines in a broad transverse suture; 

 behind this point they only appear on the palate as a narrow strip on either side. 

 Behind the last alveolus the maxilla is produced backwards into a short pointed 

 process which forms the outer wall of a foramen (PI. II. fig. 1, m.p.f.), the rest of 

 which is a deep notch in the edge of the palatine. Above this foramen the hinder 

 border of the maxilla forms the outer edge of a groove (p-P-g-)-, running up towards 

 the anterior opening of the alisphenoid canal (text-fig. 4, al.c). This groove and 

 foramen, which seem to be homologous with the posterior palatine foramen of the human 

 skull, also occur in Hyrax (Procavia) and possibly other Ungulates, and no doubt 

 transmitted branches of the palatine nerve and blood-vessels. The foramina to which 

 the name posterior palatine is applied in most mammals lie on the palate at or near 

 the junction of the anterior end of the palatines with the maxilte ; and these openings 

 are also present in Arsinoitherium, though very small. Probably both pairs of 

 openings simply mark the points at which the palatine vessels and nerves passed 

 between the maxillae and the palatines, when these bones were still separate in the 

 young ; and the presence or absence of the anterior or posterior of the foramina 

 mentioned depends on whether the palatine nerves and vessels passed out in one or 

 more groups on either side. In Arsinoitherium the posterior openings are large, and 

 those near the anterior end of the palatines small, owing probably to some structural 

 peculiarity which rendered necessary a larger nerve- and blood-supply for the posterior 

 region of the palate. In some animals there is a series of small foramina on either 

 side, the anterior one usually being much the largest. 



In front of the posterior edge above mentioned the maxilla widens into a large 

 mass, which in the adult lodges the roots of the posterior molars, and in the young, in 

 which it is relatively still larger, contains the developing germs of the same teeth. 

 This alveolar mass to a considerable extent forms a floor to the orbit. Altl^ough the 

 maxilla is excluded from the actual border of the orbit by the union of the lachrymal 

 and jugal, it nevertheless forms a large part of its anterior wall in the neighbourhood 

 of the posterior opening of the antorbital canal, where it unites in suture with the 



