UNGULATA. 315 



proportions resembling the existing llamas and horses. These 

 are named LITOPTERNA (smooth heel-bone) in allusion to the 

 facette on the calcaneum for the articulation of the fibula. 

 The brain-cavity is small. The dentition is complete or nearly 

 so, with the canines either small or wanting ; the premolars 

 and molars usually have low crowns (brachyodont), though 

 some approach the hypsodont form, and are all distinctly 

 rooted. The ends of the cervical vertebrae are flattened. 

 Clavicles are absent, and there is no entepicondylar foramen 

 in the humerus. The femur bears a third trochanter. The 

 bones of the two rows in the carpus and tarsus exhibit their 

 primitive relationships, not interlocking or alternating. The 

 digits vary in number from five to one in the different genera, 

 the most usual number being three ; but in every case the 

 third digit is the largest, as in the Perissodactyla. The proxi- 

 mal end of the astragalus forms a distinct trochlear surface, 

 while its distal end is more or less convex. The calcaneum 

 articulates extensively with the fibula, thus differing from that 

 of the other odd-toed ungulates (Perissodactyla) and resembling 

 the ordinary even-toed forms (Artiodactyla). 



These animals may indeed be said to mimic the Perisso- 

 dactyla in their development; their distinctly lower grade in 

 the ungulate order being indicated by the primitive characters 

 of the brain, the teeth, the cervical vertebrae, and the carpus 

 and tarsus. It is also interesting to note that they attained 

 their maximum specialization at an earlier period than the 

 Perissodactyla; for typical fragments of them have been 

 obtained from the Pyrotherium Formation of Patagonia, while 

 important portions of the skeleton of the genera with most 

 reduced dentition and atrophied lateral digits are known 

 from the Santa Cruz Formation (e.g., Proterotherium and 

 Thoatherium). A comparatively generalized large animal, 

 Macrauchenia, with long neck and three complete digits, is 

 the latest representative of the group in the Pampa Formation 

 (Pleistocene) of Buenos Aires, and in corresponding deposits in 

 Bolivia. 



Proterotherium (fig. 180c). The rostral part of the skull is short and 

 tapering, and the nasal bones are elongated. The orbit is completely 



