UNGULATA. 311 



noted that the carpus loses its os centrale, while the hind foot iw 

 deprived of one digit. In the early forms with five digits on 

 each foot, the inner digit (no. i) so far as known is opposable to 

 the others. 



Pachyrucus. A small animal known by nearly complete skeletons 

 from the Santa Cruz and Lower Pampa Formations of the Argentine 

 Republic. The shape of the skull is much like that of a rabbit, but the 

 inflated auditory bullae are exposed on the roof of the occipital region. 

 The zygomatic arch is large, but the orbital cavity is not separated from 

 the temporal fossa. The condyles of the mandible are rounded or a little 

 elongated antero-posteriorly, and the rami are fused together at the 



. . i. 1, c. 0, pm. 3, m. 3 



svmphysis. The dental formula is .-- ; - s , and all the 



i. 2, c. 0, pm. 3, m. 3 



teeth are hypsodont, without roots, and must thus have been persistently- 

 growing. The upper incisors are much extended along the margin of the 

 jaw, the width of each equalling that of the two opposed lower incisors. 

 The molars and premolars are much laterally compressed, and those of 

 the upper jaw do not exhibit a fold on the inner face. The vertebral 

 column is remarkably rodent-like, and the lumbar vertebrae bear broad 

 descending transverse processes. The radius and ulna are free, but the 

 tibia and fibula are fused both proximally and distally. The pelvis is 

 notably elongated. 



Typotherium (fig. 178). The skull in this genus is shaped much like 

 that of Pachyrucits, and the orbital cavity is similarly not separated from 

 the temporal fossa ; but the auditory bullse are scarcely inflated and are 

 confined to the lower part of the occipital region. The dentition is slightly 



i. I.e. 0, pm. 2, m. 3 



more reduced, the formula being . _ _ ; and the enamel on 



i. 2, c. 0, pm. 1, m. 3 



the inner face of the upper molars exhibits two vertical folds. The cervical 

 vertebrae are short, and the lumbars bear nearly horizontal transverse 

 processes. The sacrum is very long, comprising seven vertebrae, and the 

 pelvic elements are also much elongated. The tail is short and small. 

 The femur bears a large third trochanter ; while the tibia and fibula, like 

 the radius and ulna, are entirely free. The fore foot is five-toed with 

 slightly cleft ungual phalanges ; the hind foot is four-toed, with hoof-like 

 ungual phalanges. The typical species, Typotherium cristatum, from the 

 Pampa Formation of Buenos Aires, is one of the best-known and largest 

 forms, with a skull about 0*3 m. in length. 



Sub-Order 7. Toxodontia. 



Though much resembling the Typotheria, the TOXODONTIA 

 (thus named from the typical genus Toxodon) are more normal 

 ungulates (i.) in being destitute of clavicles, and (ii.) in wanting 



