EVOLUTION OF THE PROBOSCIDEA. 



117 



(8) The humerus of Mosritherium is extremely like that of a Sirenian. Many of 

 the above-mentioned characters are by themselves of little import, but the coincidence 

 of so many points of similarity seem to render the existence of a common ancestry for 

 the two groups at least probable. 



Summary. 



The changes undergone by the skull, mandible and dentition in the Proboscidea in 

 passing from the Eocene to the recent types, may be summarised as follows :— 



The Skull. — Owing to the increase in the size of the tusks and to the presence of 

 the proboscis the facial region of the skull becomes shortened, and at the same time 

 the premaxillas become wider. The presence of the proboscis also accounts for the 

 position of the external nares. The demand for a greater surface of attachment for 

 the muscles, supporting a skull rendered heavy by the tusks and trunk, is met by the 

 great development of the diploe in certain of the cranial bones, resulting in the 

 enormous expansion of the forwardly sloping occipital surface. The maxillse become 

 greatly enlarged pari passu with the increase in the size and degree of hypselodonty 

 of the molars. At the same time the zygomatic arch becomes weaker and the jugal 

 takes a smaller share in its composition. 



The Mandible. — The mandible is at first short and stout with a massive symphysis. 

 Afterwards it becomes more and more elongated as the stature of the animals 

 increases ; the elongation is for the most part effected by the lengthening of the 

 symphysial region, but the rotation backward of the ascending ramus tends to the 

 same end. The prolongation of the mandible beyond the premaxillse must have been 

 covered by a proboscis-like structure composed of the upper lip and nose, probably 

 more or less prehensile at its extremity. The lengthening of the mandible seems to 

 have reached its maximum degree in the Middle Miocene, after which for some reason 

 or other it again became shortened by the reduction of the symphysis, while the fleshy 

 and now mobile proboscis was left behind as the sole organ of prehension. 



The Dentition. — In the upper jaw the chief changes are the loss of I. 1 and I. 3, 

 and the great increase in size of I. 2, which finally forms the great tusk characteristic 

 of the later Proboscidea. The canines are soon lost. In the early forms some at 

 least of the milk-molars are replaced by premolars in the usual manner, and these 

 teeth remain in wear simultaneously with the true molars, but in later forms no 

 vertical succession takes place, and as the milk-teeth are worn they are shed, being 

 replaced from behind by the forward movement of the molars. Of these also 

 the anterior may be shed, until at length, in old individuals of the later types, the last 

 molar is alone functional. The gradual increase in the complexity of the Proboscidean 

 molars is one of their most striking characteristics. All stages are to be traced 

 between the simple brachydont bilophodont (quadritubercular) molars of Moeritherium, 



