220 



direction of the articular surfaces : those of the anterior processes look- 

 ing inwards and a little upwards ; those of the posterior outwards and a 

 little downwards. 



In the Macrauchenia a small longitudinal process is given off immedi- 

 ately below the base of the anterior oblique process ; this structure is 

 not observable in any of the cervical vertebrae of the Giraffe or Camel 

 tribe. In the form of the articulating surfaces of the bodies of the ver- 

 tebrae the Macrauchenia deviates from the Giraffe and Camel but re- 

 sembles the Llama and Vicugna. In the Giraffe and Camel the anterior 

 articulating surface is convex and almost hemispheric. The posterior 

 surface is proportionally concave, so that the cervical vertebrae are 

 articulated by ball and socket joints ; yet not as in most reptiles 

 with intervening synovial cavities, but by the concentric ligamen- 

 tous intervertebral substance characteristic of the mammiferous class. 

 In the Llama and Vicugna the degree of convexity and concavity in 

 the articular surface of the bodies of the cervical vertebrae is much less 

 than in the Camels ; and consequently they carry their necks straighter 

 and more erect. In the Macrauchenia the anterior articulating surface 

 presents a still slighter convexity than in the Llama, and the posterior 

 surface presents a correspondingly shallower concavity. The form of 

 the extremities of the body of the vertebrae, especially of the posterior, 

 is subhexagonal, the breadth being to the depth as eight to five. 



The sides and under part of the vertebrae are slightly concave ; on the 

 inferior surface there are two ridges continued forwards from the poste- 

 rior margin of the vertebra, each situated about an inch distant from the 

 middle line ; they converge as they pass forwards and are gradually lost 

 in the level of the vertebra ; their greatest elevation does not exceed 

 half an inch. In the Aucheniae there is a longitudinal protuberance in 

 the mesial line, instead of the two ridges. The two long cervical 

 vertebrae of the Macrauchenia are also characterised by the maintenance 

 of an almost uniform diameter of the body, both in its vertical and 

 transverse extent : the cervical vertebrae of the Vicugna come nearest to 

 them in this respect : those of the Camel deviate further in the large ex- 

 cavation at the under part of the body. 



