314 MAMMALIA. 



compressed, with three internal folds. The upper incisors are scalpriform 

 or chisel-shaped, but the tw inner pairs of the lower jaw are directed 

 straight forwards. The premolars are preceded by milk-molars with well- 

 formed roots, while the incisors and canine also seem to have had de- 

 ciduous predecessors. The trunk is of very robust proportions, and the 

 back curves downwards in front to the lowly-placed neck and head. The 

 cervical vertebrge are short, with flattened ends. The dorso-lumbar region 

 comprises 20 vertebrae, of which the anterior dorsals bear large neural 

 spines. The caudal vertebrae are much flattened dorso-ventrally. The 

 fore limbs are shorter than the hind limbs. The scapula bears a very 

 large spine ; the humerus is not perforated above its lower end ; the ulna 

 has a large olecranon process, while the radius crosses this bone (as in the 

 Proboscidea) articulating only with the outer half of the distal surface of 

 the humerus ; the carpus exhibits the characteristic alternation of the 

 bones in its two series, while the three digits (n, in, iv) are short and 

 stout. The ilium is a much expanded bone ; the femur is destitute of a 

 third trochanter ; the tibia and fibula are anchylosed at their extremities, 

 while the latter bone articulates considerably with the calcaneum ; the 

 three digits, like those of the fore foot, are short and stout. No dermal 

 armour has been observed. The typical species T. platensis (fig. 179) has 

 a skull about O7 m. in length, and is found in the Pampa Formation 

 (Pleistocene) of Buenos Aires. 



Nesodon. An early genus of Toxodonts, comprising species of small 

 or moderate size, with the limbs (and probably also the neck) more slender 

 than in the typical genus. The nasal bones extend considerably forwards, 

 so that the narial opening is directed anteriorly. A cast of the brain- 

 cavity shows that the cerebral hemispheres were large, much convoluted, 

 and distinctly overlapping the cerebellum. The teeth are arranged 

 in an almost continuous series, and in the adult all are distinctly 

 rooted except upper i. 2 and lower i. 3. The formula of the milk 



dentition is .'-- , while that of the permanent dentition is 



i. 3, c. 1, m.-rn. 4 



i. 3, c. 1, pm. 4, m. 3 



.5 = -: : and the change from one dentition to the other is 



i. 3, c. 1, pm. 4, m. 3 ' 



so gradual that it is difficult to identify the various teeth in skulls of 

 different degrees of maturity. The trunk is imperfectly known ; but the 

 femur is proved to have been provided with a small third trochanter, while 

 the slender tridactyl feet must have been digitigrade. All the known 

 species were obtained from the Santa Cruz Formation of Patagonia, and 

 the typical form, N. imbricatus, has a skull about half a metre in length. 



Sub-Order 8. Litopterna. 



The third group of primitive extinct ungulates peculiar 

 to South America, comprises digitigrade animals in general 



