78 BAHIA BLANCA. [CHAP. v. 



shells, of which thirteen are recent and four others very closely 

 related to recent forms.* From the bones of the Scelidotherium, 

 including even the knee-cap, being intornbed in their proper 

 relative positions, and from the osseous armour of the great arma- 

 dillo-like animal being so well preserved, together with the bones 

 of one of its legs, we may feel assured that these remains were 

 fresh and united by their ligaments, when deposited in the gravel 

 together with the shells, f Hence we have good evidence that the 

 above enumerated gigantic quadrupeds, more different from those 

 of the present day than the oldest of the tertiary quadrupeds of 

 Europe, lived whilst the sea was peopled with most of its present 

 inhabitants; and we have confirmed that remarkable law so often 

 insisted on by Mr. Lyell, namely, that the " longevity of the species 

 in the mammalia is upon the whole inferior to that of the testacea."t 

 The great size of the bones of the Megatheroid animals, includ- 

 ing the Megatherium, Megalonyx, Scelidotherium, and Mylodon, 

 is truly wonderful. The habits of life of these animals were a 

 complete puzzle to naturalists, until Professor Owen solved the 

 problem with remarkable ingenuity. The teeth indicate, by their 

 simple structure, that these Megatheroid animals lived on vegetable 

 food, and probably on the leaves and small twigs of trees ; their 

 ponderous forms and great strong curved claws seem so little 

 adapted for locomotion, that some eminent naturalists have actually 

 believed, that, like the sloths, to which they are intimately related, 

 they subsisted by climbing back downwards on trees, and feeding 

 on the leaves. It was a bold, not to say preposterous, idea to con- 

 ceive even antediluvian trees, with branches strong enough to bear 

 animals as large as elephants. Professor Owen, with far more 

 probability, believes that, instead of climbing on the trees, they 



* Since this was written, M. Alcide d'Orbigny Las examined these 

 shells, and pronounces them all to be recent. 



t M. Aug. Bravard has described, in a Spanish work (' Observaciones 

 Gcologicas,' 1857), this district, and he believes that the bones of the 

 extinct mammals were washed out of the underlying Pampean deposit, 

 and subsequently became embedded with the still existing shells ; but I 

 am not convinced by his remarks. M. Bravard believes that the whole 

 enormous Pampean deposit is a sub-aerial formation, like sand-dunes: 

 this seems to me to be an untenable doctrine. 



t Principles of Geology, vol. iv. p. 40. 



This theory was first developed in the Zoology of the Voyage of the 

 Beagle, and subsequently in Professor Owen's 'Memoir on Mylodon 

 robustus. J 



