155 



fered with its rapid and frequent movements ; and the same dimensions of the 

 jaw which provided space for the ever-active matrix of the deeply implanted 

 teeth, gave the required capacity to the cavity of the mouth when the tongue 

 was retracted and at rest. 



The Megatherium whose teeth and jaws were adapted to the comminution of 

 coarser parts of the foliage of trees, appears to have had the advantage of a 

 short proboscis, as an adjunct to the tongue, in the work of stripping off the 

 smaller branches of the prostrate tree ; and since, in proportion as the lips and 

 nose were modified to gain prehensile power, an inordinate development of 

 tongue would be less needed, the evidence of the proboscis in the Megatherium 

 harmonizes with the smaller size of its hypoglossal nerves, and with the dimi- 

 nution of the capacity of its mouth, occasioned by the narrowing of the palate 

 and the mutual approximation of the lateral series of grinders. The Elephant, 

 the hugest of existing phyllophagous quadrupeds, is characterized by a maximized 

 proboscis ; the Giraffe by a long and muscular tongue. Both these characteristics 

 coexisted in the Megatherium, the size of the proboscis being diminished. ' In 

 the Mylodon, which had no proboscis, the compensation was the more largely 

 developed tongue, and it thus offers, in respect of the mechanism for stripping 

 off foliage, a striking contrast with the almost tongueless Elephant. 



The analogy of existing animals thus teaches us that the modifications of 

 such soft and perishable parts as the tongue and nose, of which we obtain evi- 

 dence from the enduring remains of the extinct Megatherioids, are in strict ac- 

 cordance with the theory of their subsisting on foliage, and of their uprending 

 trees to obtain it ; but it does not explain the utihty of a prehensile tongue or 

 proboscis to such animals, on the supposition that they subsisted on roots. 



There is yet another peculiarity in the cranial organization of the Megatherian 

 animals that harmonizes with habits of life which rendered them unusually ob- 

 noxious to blows from falling bodies, and may even be conceived to have been 

 a designed modification in relation to those habits : I allude to the extensive air- 

 cells introduced between the external and vitreous tables of the skull ; and I pro- 

 pose in this place to inquire into the probable cause of the fractures, from the 

 immediately fatal effects of which the subject of the present Memoir appears to 

 have been saved by virtue of this remarkable structure. 



u2 



