106 BAIIIA BLANCA. 



sea might now wasli up on a shallow bank. They 

 were associated with twenty-three species of shells, 

 of which thiiteen are recent and four others very 

 closely related to recent forms ; whether the re- 

 maining ones are extinct or simply unknown, must 

 be doubtful, as few collections of shells have been 

 made on this coast. As, however, the recent spe- 

 cies were embedded in nearly the same propor- 

 tional numbers with those now living in the bay, I 

 think there can be little doubt that this accumu- 

 lation belongs to a very late tertiary period. From 

 the bones of the Scelidotherium, including even the 

 knee-cap, being intombed in their proper relative 

 positions, and from the osseous armour of the great 

 armadillo-like animal being so well preserved, to- 

 gether with the bones of one of its legs, we may 

 feel assured that these remains were fresh and uni- 

 ted by their ligaments when deposited in the grav- 

 el together with the shells. Hence we have good 

 evidence that the above enumerated gigantic quad- 

 rupeds, 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 con- 

 firmed that remarkable law so often insisted on by 

 Mr. Lyell, namely, that the "longevity of the spe- 

 cies in the mammalia is, upon the whole, inferior to 

 that of the testacea."* 



The great size of the bones of the Megatheroid 

 animals, including the IMegatherium, Megalonyx, 

 Scelidotherium, and Mylodon, is truly wonderful. 

 The habits of life of these animals were a complete 

 puzzle to naturalists, until Professor Owenf lately 

 solved the problem with remarkable ingenuity. 



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



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

 of the Beagle, and subsequently in Professor Owen's Memoir on 

 Mylodon robustus. 



