In the more recent forms of Typotheria the dentition has become reduced. The canines are lost, and as the incisors are reduced also, to one on each side of the upper, and two on each side of the lower jaw, the likeness to a Rodent skull is increased. There is also evidence of a modification from the more primitive forms in the loss of one premolar or even more, in the alternating bones of the carpus, in the disappearance of the centrale, and in the loss of a toe upon the hind-foot. In these more recent forms the fibula articulates with the astragalus instead of with the calcaneum. Typotheria of these more recent forms may be illustrated by the typical genus _Typotherium_. It has the reduced dental formula I 1/2 C 0/0 Pm 2/1 M 3/3; the molars are simple in pattern, and much like those of _Toxodon_. The upper incisors are powerful and curved, but are surrounded by a layer of enamel, which is not limited to the anterior face, as it practically is in Rodents. The sacrum is composed of a large number of vertebrae--some seven--a state of affairs which recalls the Edentata. The shoulder blade is not Ungulate in form. It has a strong spine, with an acromion and a well-developed metacromion. The terminal phalanges are enlarged and hoof-like. In the genus _Pachyrucos_ there are three premolars, otherwise {214} the formula is the same as in _Typotherium_. The animal seems to have had nails rather than hoofs. The thumb was opposable. The fibula is fused below with the tibia, whereas in the last genus these two bones are quite separate from each other. SUB-ORDER 5. TOXODONTIA The group Toxodontia,[135] like so many others, is exceedingly hard to define. Nor are its limits any easier to mark out than many others of the groups of Ungulates. It will be best perhaps to give an account of _Toxodon_, and of a few types which seem to lie near it in the system, and then to indicate how far they resemble or depart in structure from other Ungulates. _Toxodon_ itself is known from complete skeletons. It lived in Argentina during the "Pampean" period, which seems to be of the Pleistocene age. A large number of species, however, have been described, some of which seem to go farther back in time, and to have existed during the Miocene period further south in Patagonia.