TOXODONTIA. An extinct order or sub-order of South American hoofed mammals, some (toxodonts) being massively proportioned three-toed beasts which played the part of rhinocer oses, and others (typotheres) small animals more nearly compar able to rabbits and the larger rodents in appearance and probably in habits. The order was distinguished by long-crowned, often rootless teeth of peculiar pattern, the front teeth enlarged and somewhat like those of rodents, the cheek teeth of triangular pattern above and unequal crescents below, the jaws deep and rather short, heavy zygomatic arches and the mastoid bone pe culiarly inflated. The feet have from three to five digits, the ar rangement of the carpals is alternating but the astragalus has a short neck and convex head, the fibula articulates with the cal caneum but the navicular does not reach the cuboid. In the Miocene of Patagonia the toxodonts are of moderate size, Nesodon as large as a tapir, Adinotherium of the size of a sheep, and the ty potheres are all small creatures comparable with a wood-chuck or guinea-pig. In the Pleistocene of Argentina Toxodon equals the Indian rhinoceros in size, Toxodontherium and Xotodon of the Pliocene are no less gigantic, while Typotherium, the largest of the smaller group, may be compared to the modern capybara.
A third assemblage, the Entelonychia, is related to the toxo donts and typotheres and included with them under the order Notoungulata (also called Toxodonta). They are more primitive
in teeth and foot-construction, many of them brachyodont, five toed, clawed and chiefly found in the older Tertiary formations of South America, with a few specialized survivors in the Miocene. Among these last Homalodontotherium, with large compressed claws but with grinding teeth somewhat like the rhinoceros type, is analogous to the Chalicotheriidae of the northern world, while Astrapotherium shows a remarkably close parallelism to the Met amynodon of the north. The Litopterna (q.v.) also have some affinity to the notoungulate groups, the whole assemblage being peculiar to South America save for a couple of small and primitive entelonychians found in the Lower Eocene of Wyoming (Arctos tylops) and the Paleocene of Mongolia (Palaeostylops) ; these may indicate that these South American Tertiary "ungulates" originally came from Holarctica about the end of the Cretaceous.
The invasion of South America by northern mammal faunas at the end of the Tertiary period brought about the progressive ex tinction of the toxodonts and their allies, Toxodon and Macrau chenia being the last survivors. Unlike the edentates, the South American hoofed animals do not seem to have made any counter invasion into North America. (W. D. M.)