in form, and in the direction o f the articular surfaces ; those o f the anterior proc
e sse s looking inwards and a little upwards ; those o f the posterior, outwards and
a little downwards.
In the Macrauchenia a small longitudinal process (c, fig. 2, PI. V I I .) is given
o ff immediately below the base o f the anterior oblique process ; this structure is
not observable in any o f the cervical vertebræ o f the Giraffe or Camelidæ.
In the form o f the articulating surfaces o f the bodies o f the vertebræ the
Macrauchenia deviates from the Giraffe and Camel, but resembles the Aucheniæ.
In the Giraffe and Camel the anterior articulating surface is convex and almost
hemispheric, the posterior surface is proportionally concave, so that the cervical
vertebræ are articulated b y ball and so ck et joints ; y e t not, as in most Reptile s,
with intervening synovial cavities, hut hy the concentric ligamentous intervertébral
substance characteristic o f the Mammiferous c la ss. In the Llama and
Vicugna, the degree o f convexity and concavity in the articular surface o f the
bodies o f the cervical vertebræ is much le ss than in the Camels ; and in consequence
th ey carry their n eck s more stiffly and more in a straight line. In
Macrauchenia the anterior articulating surface (fig. 2, PI. V I I .) presents a still
slighter convexity than in the Llama (fig. 4, PI. V IL ), and the posterior surface
(fig. 1, PI. V II .) presents a correspondingly shallower conca v ity . The form o f the
extremities o f the b ody o f the vertebræ, e sp e c ia lly o f the posterior, is sub-hexagonal,
the breadth being to the depth as eight to five. The sides and under part
o f the vertebræ are sligh tly concave ; on the inferior surface there are two ridges,
continued forwards from the posterior margin o f the vertebra, each situated about
an inch distant from the middle line ; they converge as they pa ss forwards, and
are gradually lo st in the level o f the vertebra ; their greatest elevation does not
exceed h a lf an inch. In the Aucheniæ there is a longitudinal protuberance in the
mesial line, instead o f the two ridges. Th e two long cervical vertebræ o f the
Macrauchenia are also characterized hy the maintenance o f an almost uniform
diameter o f the body, both in its vertical and transverse ex ten t; the ceiwical vertebræ
o f the Vicugna come nearest to them in this respect; those o f the Camel
deviate further in the large excavation at the under part o f the body.
Th e long vertebral or spinal canal offers a sligh t enlargement at the two
extremities ; this structure which is generally in the ratio o f the extent o f motion
o f the vertebræ on each other is more marked in the Camel, where the form and
mode o f articulation o f the bodies o f the vertebræ are designed to admit o f a free
and extensive inflection o f the cervical vertebræ ; and the result o f th is structure
is very obvious in the sigmoid flexure o f the neck in the living animal. In the
Aucheniæ, on the contrary, the neck is carried le ss gracefully erect and in an almost
straight line, and the form o f the vertebræ and the nature o f their joints correspond,
as we have seen, to this condition. From the length o f the bodies o f the