In all the genera the teeth number 5/4; the teeth on each side were sometimes in continuous series, sometimes the first one was isolated and almost always more or less tusk-like, most so in †Eucholœops. The other teeth were usually of transversely elliptical shape and worn into two ridges, with a hollow between; the †mylodonts (†Nematherium, etc.) already had the triangular, or lozenge-shaped, lobate form of teeth, characteristic of the family.
The skull varied considerably in its proportions; generally, it was long and narrow, with shortened face and elongate cranium; the sagittal crest was seldom present, never prominent, and the orbit was always widely open behind, without postorbital processes. The premaxillaries were always short and toothless and in most of the genera they were slender rods, in others (e.g. †Hyperleptus) broad and plate-like. The lower jaw had an elongate spout-like symphysis, in which the two halves were coössified, tapering forward to a blunt point and, though the length of this spout differed greatly in the various genera, in none was there a broad, abrupt chin such as †Mylodon and †Megalonyx had. In †Prepotherium, which is believed to be referable to the †Megatheriidæ, the lower jaw had the extremely convex inferior border, in less exaggerated degree, of its huge Pampean successor; it would be premature to say descendant.
While the long, slender skull was the prevailing type among the Santa Cruz †Gravigrada, there was a group of small animals in which the skull was shorter and more rounded and had a very suggestive likeness to that of the modern tree-sloths, as was likewise true of the teeth.
Despite innumerable variations of detail, the skeleton of the Santa Cruz †ground-sloths may be described without distinction of genera, though it should be added that the skeleton is but partially known in many of the genera, and fuller knowledge might require modification of some of the statements. The neck was of moderate length, the body long, the tail long and heavy and, in some instances, very massive. The sternal ribs were completely ossified and already had the same elaborate mode of articulation with the breast-bone as in the great Pampean forms, and the vertebræ the same intricate connections. The shoulder-blade also had the same characteristics as in the latter, but the hip-bones had but a moderate transverse expansion, having no huge mass of viscera to support.
The limbs were stout and short, fore and hind legs of nearly equal length; the humerus had the epicondylar foramen and the broad, flattened femur retained the third trochanter. The radius had a discoidal upper end, which rotated freely upon the humerus; the tibia and fibula were always separate. The feet were five-toed, all the digits complete and functional and all provided with claws; there was no coössification between the phalanges. The astragalus was little different from the normal form, but in some genera (e.g. †Prepotherium) the highly peculiar form of this bone characteristic of †Mylodon and †Megatherium was distantly foreshadowed. The gait must have been simply plantigrade, though some of the forms had probably begun to throw the weight upon the outer edge of the foot.
Fig. 290.—Left pes of †Mylodon, Pampean (after Owen). Cal., calcaneum. As., astragalus. N., navicular. Cn. 2, Cn. 3, middle and external cuneiforms. Cb., cuboid.
No dermal armour has yet been found in association with any of the genera, and, so far as the predominant †Megalonychidæ are concerned, of which so many skeletons have been collected, this negative evidence must be allowed great weight. But the material of the other two families is so rare and incomplete, that the failure to find dermal ossicles is of no value in determining the question; probably, the †mylodonts possessed them.