318 MAMMALIA. 



surrounded by bone, and thus separated from the temporal fossa. The 



i. 1, c. 0. pm. 4, m. 3 m , . . 



dental formula of the adult is ^. - _ . The upper incisors 



i. 2, c. 1, pm. 4, m. 3 



are a little enlarged, and the outer lower incisors are larger than the inner 

 pair. The lower canine is insignificant. The grinding teeth exhibit a 

 general resemblance to those of Palceotherium (p. 326), and pm. 3, 4, are 

 almost as complex as the true molars. The hindermost lower molar ex- 

 hibits a third lobe. The neck is comparatively short, and the vertebrarterial 

 canal is normal, not penetrating the arches of the cervical vertebrae as in 

 Macrauchenia. The third trochanter of the femur is well developed. Each 

 foot (fig. 180c) is tridactyl, with the lateral digits much reduced though 

 complete. The genus occurs only in the Santa Cruz Formation of Pata- 

 gonia, and all the species are of small size, the skull in Proterotherium 

 cavum attaining a length of about 0'16 m. 



Thoatherium (figs. 180 D, E). Another small animal with nearly com- 

 plete dentition. The two pairs of lower incisors are about equal in size, 

 and the hindermost lower molar is destitute of a third lobe. Both feet are 

 monodactyl like those of the existing horses. The lateral metapodial bones 

 are even more reduced than in these more modern ungulates, but in the 

 hind foot the astragalus articulates only with the navicular, not extending 

 over the cuboid (fig. 180 E). Thoatherium crepidatum and other species 

 occur in the Santa Cruz Formation of Patagonia. 



Macrauchenia. The skeleton of this genus is almost completely 

 known. The skull is long and narrow, with the premaxillo-maxiliary 

 region much extended and the nasal bones rudimentary, so that the narial 

 opening is situated far back and directed upwards. The disposition of 

 the parts may indicate the original presence of a small proboscis. Behind 

 the narial opening there are deep pits in the frontal region, probably 

 for muscular attachments. The orbit is completely surrounded by bone, 

 and thus separated from the temporal fossa. The dentition is complete, 



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



namely, .--- - , in a regular close series, and the teeth are 



<7 ' i. 3, c. 1, pm. 4, m. 3' 



somewhat deepened (hypsodont). The ordinary succession.al teeth are 

 known. The cervical vertebrae are large and much elongated, and the 

 canal for the vertebral artery does not perforate their transverse processes, 

 but passes obliquely through the pedicle of the neural arch (as in the 

 camels, llamas, and Myrmecophaga). The dorso-lumbar vertebrae are about 

 24 in number. In the fore limb the radius and ulna are fused together, 

 and the olecranon process of the latter is small. In the hind limb the 

 femur bears a small third trochanter, while the fibula is an excessively 

 slender bone partially fused with the tibia. Both feet (like those of 

 Theosodon shown in figs. 180 A, B) are tridactyl, with the median digit not 

 much larger than the lateral digits. The typical and best known species 

 is Macrauchenia patachonica, about as large as a camel, with the skull half 

 a metre in length. This occurs in the Pampa Formation (Pleistocene) of 



