FOSSIL MAMMALIA. 



39 



in form, and in the direction of the articular surfaces ; those of the anterior pro- 

 cesses looking inwards and a little upwards ; those of the posterior, outwards and 

 a little downwards. 



In the Macrauchenia a small longitudinal process (c, fig. 2, PI. VII.) is given 

 off immediately below the base of the anterior oblique process ; this structure is 

 not observable in any of the cervical vertebras of the Giraffe or Camelidae. 



In the form of the articulating surfaces of the bodies of the vertebrae the 

 Macrauchenia deviates from the Giraffe and Camel, but resembles the Aucheniae. 

 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 ligamentous interver- 

 tebral 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 in conse- 

 quence they carry their necks more stiffly and more in a straight line. In 

 Macrauchenia the anterior articulating surface (fig. 2, PI. VII.) presents a still 

 slighter convexity than in the Llama (fig. 4, PI. VII.), and the posterior surface 

 (fig. 1, PI. VII.) presents a correspondingly shallower concavity. The form of the 

 extremities of the body of the vertebrae, especially of the posterior, is sub-hexa- 

 gonal, 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 posterior 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 characterized by the maintenance of an almost uniform 

 diameter of the body, both in its vertical and transverse extent; the cervical verte- 

 brae of the Vicugna come nearest to them in this respect; those of the Camel 

 deviate further in the large excavation at the under part of the body. 



The long vertebral or spinal canal offers a slight enlargement at the two 

 extremities ; this structure which is generally in the ratio of the extent of motion 

 of the vertebrae on each other is more marked in the Camel, where the form and 

 mode of articulation of the bodies of the vertebrae are designed to admit of a free 

 and extensive inflection of the cervical vertebrae ; and the result of this structure 

 is very obvious in the sigmoid flexure of the neck in the living animal. In the 

 Aucheniae, on the contrary, the neck is carried less gracefully erect and in an almost 

 straight line, and the form of the vertebrae and the nature of their joints corre- 

 spond, as we have seen, to this condition. From the length of the bodies of the 



