1903.] 



On the Evolution of the Proboscidea. 



443 



"On the Evolution of the Proboscidea." By C. W. Andrews, 

 D.Sc. Communicated by Professor E. Pay Lankestee, F.E.S. 

 Peceived March 5,— Pead March 26, 1903. 



(Abstract.) 



Until the author's recent discoveries of primitive Proboscidea in the 

 Middle and Upper Eocene formations of the Fayum, Egypt, the oldest 

 known members of this mammalian order were Dinotherium cuvieri and 

 Tetrabehdon angustidens, from the base of the Miocene in France. The 

 new Egyptian fossils not only reveal for the first time the early 

 history of the order, but also provide more satisfactory material for 

 the discussion of its evolution than has hitherto been available. 



The most important changes in the Proboscidea occur in the skull,, 

 mandible, and dentition. 



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 premaxillse 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 cliploe in certain of the cranial bones,, 

 resulting in the enormous expansion of the forwardly sloping occipital 

 surface. The maxillae become greatly enlarged concomitantly 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 is at first short and stout, with a massive symphysis. 

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

 animals increases ; and this elongation is for the most part effected 

 by the lengthening of the symphysial region, though the backward 

 rotation of the ascending ramus tends to the same end. The pro- 

 longation 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 length- 

 ening of the mandible seems to have reached its maximum degree in 

 the Middle Miocene, after which 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. 



In the upper dentition the chief changes are the loss of incisors 

 Nos. 1 and 3, and the great increase in size of incisor No. 2, which 

 eventually forms the great tusk characteristic of the later Proboscidea. 

 The canines are soon lost. In the earliest forms, some at least of the 

 cheek-teeth (milk-molars) are replaced by premolars in the usual 



