On the manus, the three inner digits have powerful claws. This animal, too, was Pleistocene in time. The Megatheriidae had, however, small as well as gigantic forms.
The genus Zamicrus had a skull no bigger than that of a Sloth, while Nothrotherium was also a comparatively small creature; the teeth of the latter genus are reduced to 4/3.
The extinct group of the Glyptodontidae comprises large creatures with a dense covering of bony scutes which are arranged in a tesselated fashion, and thus form an immobile armature of immense strength. In correspondence with this massive carapace the dorsal vertebrae have fused together, and the lumbar vertebrae form a series ankylosed to each other and to the following sacrals. These creatures are all South American.
Fig. 106.—Glyptodon clavipes. × 1⁄12. (After Owen.)
Glyptodon, the genus which gives its name to the family, is known from numerous remains in South America, and also from so far north as Texas and Mexico. It grew to be as long as 16 or 17 feet. In the skull there is an exceedingly long downward process of the zygomatic arch, as in Sloths, the arch itself being complete. The process extends so far down as to reach a point about on a level with the middle of the lower jaw. The nasals are short or rudimentary. As in Myrmecophaga, the pterygoids enter into the formation of the bony palate. The lower jaw has a spout-shaped extremity, and, behind, it rises into an enormous vertical branch as high as the front part of the jaw is long. There are eight teeth in each half of each jaw. As in
some Armadillos, the cervical vertebrae are at least partly fused. The atlas is free, but the rest, or at any rate five of them, are united. The last cervical is sometimes fused with the succeeding dorsals; the latter are twelve in number, and are fused together so far as concerns their centra and neural processes. The succeeding region of the vertebral column includes seven to nine lumbars, which are fused with the eight sacrals; in this region the neural processes are high, and there is thus produced a strong and lofty ridge along the back, which forms a powerful support for the carapace. The fore-limbs are shorter than the hind-limbs, which latter are attached to an unusually massive pelvis. The claws of the limbs are blunt and almost hoof-like.
The heavy carapace consists of sculptured, five or six-sided plates, which have no particular arrangement in the middle, but towards the margins show indications of an arrangement in transverse rows. The moderately long tail is also encircled by bony skin-plates which are thorny above, or at least provided each with a blunt upstanding process. It appears that outside this bony system of scutes were horny epidermic scales, corresponding exactly with the tesserae which they cover. There are apparently a good many species of Glyptodon.
In the allied genus Panochthus the tail is rather longer, and the bony rings which surround it, instead of being all movable as in Glyptodon, are at first so, but later, i.e. towards the end of the tail, become welded into a single and massive piece. Both feet are here four-toed, while in Glyptodon the hind-feet are five-toed and the fore-feet four-toed.
Daedicurus shows a further specialisation, in that the feet have three and four digits respectively. The orbit too shows a specialisation in being separated from the temporal fossa. The descending process of the zygomatic arch is not so extraordinarily exaggerated as it is in Glyptodon. It has the same terminal tube of osseous scutes upon the tail. This creature seems to have reached a length of about twelve feet.