The country in which the Megatherium has been found is described by Mr. Darwin as belonging to the great Pampean formation, which consists partly of a reddish clay and in part of a highly calcareous marly rock. Near the coast there are some plains formed from the wreck of the upper plain, and from mud, gravel, and sand thrown up by the sea during the slow elevation of the land, as shown by the raised beds of recent shells. At Punta Alta there is a highly-interesting section of one of the later-formed little plains, in which many remains of these gigantic land-animals have been found. These were, says Mr. Darwin:--“First, parts of three heads and other bones of the Megatherium, the huge dimensions of which are expressed by its name. Secondly, the _Megalonyx_, a great allied animal. Thirdly, the _Scelidotherium_, also an allied animal, of which I obtained a nearly perfect skeleton: it must have been as large as a rhinoceros; in the structure of its head it comes, according to Professor Owen, nearest to the Cape Anteater, but in some other respects it approaches to the Armadilloes. Fourthly, the _Mylodon Darwinii_, a closely related genus, of little inferior size. Fifthly, another gigantic edental quadruped. Sixthly, a large animal with an osseous coat, in compartments, very like that of an armadillo. Seventhly, an extinct kind of horse. Eighthly, a tooth of a pachydermatous animal, probably the same with the Macrauchenia, a huge beast with a long neck like a camel. Lastly, the Toxodon, perhaps one of the strangest animals ever discovered; in size it equalled an Elephant or Megatherium, but the structure of its teeth, as Professor Owen states, proves indisputably that it was intimately related to the Gnawers, the order which, at the present day, includes most of the smallest quadrupeds; in many details it is allied to the pachydermata; judging from the position of its eyes, ears, and nostrils, it was probably aquatic, like the Dugong and Manatee, to which it is allied. How wonderfully are the different orders--at the present time so well separated--blended together in different points in the structure of the Toxodon!”[101] [101] “Journal of Researches,” &c., by Charles Darwin, p. 81. [Illustration: Fig. 190.--Lower jaw of the Mylodon.] The remains on which our knowledge of the _Scelidotherium_ is founded include the cranium, which is nearly entire, with the teeth and part of the os hyoides, seven cervical, eight dorsal, and five sacral vertebræ, both the scapulæ, and some other bones. The remains of the cranium indicate that its general form was an elongated slender compressed cone, beginning behind by a flattened vertical base, expanding slightly to the cheek-bone, and thence contracting to the anterior extremity. All these parts were discovered in their natural relative positions, indicating, as Mr. Darwin observes, that the gravelly formation in which they were discovered had not been disturbed since its deposition.