THE TOXODONTS 1151 



In the lower jaw the molars form nearly simple crescents, very similar to those of 

 rhinoceroses, but the last crescent of the third of the series is more elongated. 



The dentition may apparently be represented by the formula, i\, c\, p\, m\\ 

 the premolars being separated from the incisors or canines by a long gap. 



The front of the apex of the upper tusk is worn to an oblique facet by the 

 attrition of the lower canine. In the lower jaw the tusk is considerably smaller 

 than in the upper jaw, but is still triangular in section, although with the sharp edge 

 in front. The inner surface is concave anteriorly and convex posteriorly; while the 

 outer one is wholly convex, and passes imperceptibly into the small hinder surface. 

 The extremity of the latter is worn into a long oblique facet, in the same manner as 

 in the corresponding tooth of a peccary. The six lower incisors are inclined for- 

 ward, and arranged in a circle so as to fill up the interval between the tusks. Their 

 crowns, which vary in shape in the two species of the genus, are short and spatu- 

 late, with the upper surface slightly concave, and the lower one convex; a deep, 

 longitudinal groove traversing the middle of each of these two surfaces, and uniting 

 in a notch in the middle of the unworn crown. The lower cheek-teeth call for no 

 special remark, as they are very similar to those of the rhinoceroses. In the lower 

 jaw, the tusks certainly correspond to the canines; and it would appear at first sight 

 that the same would hold good with those of the upper jaw, but from the analogy 

 of the proterothere it is more probable that the latter really belong to the incisor 

 series. 



THE TOXODONTS 

 SUBORDER Toxodontia 



The toxodonts may be defined as a group of more or less aberrant Ungulates 

 with tall-crowned and curved cheek-teeth, some or all of which grow from persistent 

 pulps, either permanently or during life; while at least one pair of incisors in each 

 jaw are rootless, and the third upper incisor, when present, is placed in the line of 

 the cheek-teeth. The vertebrae of the neck are short, with flattened, articular faces 

 to the bodies, and the vertebral artery piercing the transverse process in the ordinary 

 manner. The wrist (when known) is of the alternating type, while the ankle joint 

 is formed on the linear plan. The astragalus is slightly grooved on its superior 

 face, and inferiorly is like that of the Odd-Toed group, having no head for the navic- 

 ular; but the calcaneum, which is truncated inferiorly, has a large articular surface 

 for the fibula, as in the Even-Toed section. The number of toes varies from five to 

 three; the middle one being larger than either of the others, and symmetrical in 

 itself. The femur may or may not have a third trochanter. The number of trunk 

 vertebras in the typical genus is twenty, or intermediate between that of the Even 

 and Odd-Toed groups. In form the cheek-teeth of the less specialized forms ap- 

 proximate to the Odd-Toed plan of structure; and in all the genera the enamel is 

 most developed on, or even confined to, the outer sides of the cheek-teeth, although 

 there may be vertical bands on some of the other surfaces. More specialized in the 

 structure of the feet and teeth than the last group, phylogenetically the toxodonts 

 may apparently be regarded as related to the Odd-Toed Ungulates, but as retaining 



