358 MAMMALIA. 



PODA in allusion to their cushion-like feet. The skull does 

 not bear horns, and the upper incisors do not entirely dis- 

 appear, although the premolars are reduced in number and 

 more or less dispersed. The tympanic bullae are filled with 

 spongy bone. The neck is somewhat elongated and resembles 

 that of the extinct Macrauchenia (p. 318), in the circumstance 

 that the canal for the vertebral artery penetrates and traverses 

 the pedicles of the neural arches. Although the digits are 

 reduced to two on each foot and the metapodials are fused 

 together throughout the greater part of their length, the trape- 

 zoid and magnum in the carpus and the navicular and cuboid 

 in the tarsus remain distinct. It is also noteworthy that the 

 two elements of the metapodium or cannon-bone are slightly 

 divergent at the lower or distal extremity; and the articular 

 ends are not keeled like those of ordinary ruminants, but 

 bluntly rounded and smooth. 



It was for long a strange and inexplicable fact, that the 

 mammals exhibiting these unique characters in the skeleton, 

 and other equally remarkable features in the soft parts, should 

 occur only in two widely-separated regions of the globe the 

 camels in Asia and perhaps northern Africa, the llamas and 

 vicunas in South America. The explanation is now afforded 

 by the skeletons discovered in the Miocene and Pliocene strata 

 of North America. It appears that the family was gradually 

 evolved from primitive artiodactyls in this area ; that by early 

 Pliocene times the fully differentiated camel had reached India, 

 probably by some land-bridge in the north ; and that when the 

 Isthmus of Panama was upheaved at the beginning of the 

 Pliocene period, the representatives of the llamas and vicunas 

 wandered south, where they have since flourished exceedingly. 

 All the members of the family became extinct in North America 

 at the close of the Pliocene period. 



Poebroth.eri.um. An animal of slender proportions, in form and size 

 resembling a small gazelle. The facial region of the skull is much elongated 

 and laterally compressed ; and the basicranial axis is slightly bent. The 

 orbit is relatively large and not completely separated by a bony bar from 

 the temporal fossa ; the tympanic bullse are relatively large. The den- 

 tition is complete, and the dental series is regular and almost continuous, 

 with the incisors slightly spaced and only a short diastema in front of and 



