The rodents of the Pleistocene were very nearly in their modern stage of development, most of the genera and many of the species surviving to present times. Just what members of the order were introduced from the Old World, the imperfect and fragmentary history will not permit us to say, but some interesting South American immigrants should be noted. One of these, the Capybara or so-called Water-Hog (_Hydrochærus capybara_), the largest of existing rodents, failed to gain a permanent foothold, but another South American form, the Short-tailed or Canada Porcupine (_Erethizon dorsatus_), common all over the United States in the Pleistocene, has maintained itself to the present day. One especially peculiar form, not derived from South America or the Old World, is the †Giant Beaver (_†Castoroides_), one species of which, _†C. ohioensis_, was as large as a Black Bear and occurred in the later Pleistocene, while a smaller species (_†C. species indet._) is found in the more ancient deposits of the epoch. In almost all respects _†Castoroides_ was simply a gigantic beaver, but the grinding teeth were remarkably like those of the South American Capybara (_Hydrochærus_), so much so that it has been mistakenly referred to the same family by some authorities. By far the strangest elements of the Pleistocene faunas were the two suborders of gigantic edentates, the _†Gravigrada_, or †ground-sloths, and the _†Glyptodontia_, which might well be called giant armadillos, if that name were not already in use for a living Brazilian animal. Both suborders are completely extinct, but they long played a very conspicuous rôle in South America, where they originated and whence the North American representatives migrated. The †ground-sloths were great, unwieldy, herbivorous animals covered with long hair, and in one family (†Mylodontidæ) there was a close-set armour of pebble-like ossicles in the skin, not visible externally; they walked upon the outer edges of the feet, somewhat as the Ant-Bear (_Myrmecophaga_) uses his fore paws, and must have been very slow-moving creatures. Their enormous claws may have served partly as weapons of defence and were doubtless used also to drag down branches of trees and to dig roots and tubers. Apparently, the latest of these curious animals to survive was the very large _†Megalonyx_, which, it is interesting to note, was first discovered and named by Thomas Jefferson. The animals of this genus were very abundant in the forests east of the Mississippi River and on the Pacific coast, much less common in the plains region, where they would seem to have been confined to the wooded river valleys. The still more gigantic _†Megatherium_, which had a body as large as that of an elephant and much shorter, though more massive legs, was a southern animal and has not been found above South Carolina. _†Mylodon_, smaller and lighter than the preceding genera, would seem to have entered the continent earlier