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Show 38 ZOOLOGY OF TilE VOYAGE OF TilE BEAGLE. . . . f th bony canals for the vertebral arteries which are pecu-ture and disposition o e I'd g existing Mammalia. In Plate VI. . l h t . tic of the Came I re amon . bar y c arac ens d ··fi f the canal for the vertebral artery are shown m a fi 2 the groove an OJI ces 0 • • h "fi g. ' . . I· . Plate Vll. figures 1 and 3 exhibit t e on ces section exposmg the spmal cana · m · · · of the at the commencement of the arterial canals, as seen m a posterwr view . . fi 2 d 4 the terminations of the same canals are shown, m the verte~rre ;. m fgst.llesa anme 'vertebrre. the smaller figures (3 and 4) are taken from antenor view o ' 1 · 1 the fourth cervi· cal verte b ra of a Llama · The vertebrre of the MacLralu c 1em· at la s·o closely resemble the middle cervical vertebrre of the Vicugna an~ ama 111 ~elf lonaated form. approaching the Auchenial division of the Camehdre, and deviat~ ng from the tr~e Camels in the relations of the length of the b~dy o~ the vertebra to its breadth and depth, and in the much smaller siz~ of the mfenor ?rocesses. Excepting the Giraffe, there is no existing mammal whiCh possesses ce~vical v~rtebrre so long as the Macrauchenia; but the cervical vertebrre of the Guaffe, differ in the situation of the perforations for the vertebral arteries, and in the form of the terminal articular surfaces, as will be presently noticed. . Both of the cervical vertebrre of the Macmucllenia here descnbed, are. of the same size, each measures six inches and a half in extreme length, tw.o mches, ten lines in breadth, and two inches, four lines in depth. In the Guaffe a~d the Camelidre, the spinous processes are thin larninre of considerable extent 111 the axis of the vertebra, but rising to a very short distance above the level of the vertebral arch: the spinous processes have the same form in ~he ~orresponding vertebrre of the Macrauchenia, but present a still greater long1tudmal extent; they commence at the interspace of the anterior oblique processes, a.nd exte~~ to opposite the base of the posterior oblique processes; the upper margm descnbmg a gentle curve, as shown in fig. 1, Pl. VI. 'rhe transverse proc~sses als~ pr~sent the form of slightly produced, but longitudinal1y extended, lammre: then· disposition is essentially the same as in the Camelidre, but more nearly corresponds with the modifications presented by the Auchenire. The inferior transverse pr?cesses,- those which are alone developed in fish, but which are not present 111 any other vertebrre save the cervical, in mammalia,-these processes in t~e Macrauchenia are continued from the sides of the under surface of the antenor part of the body of the vertebra; their extremities being broken off, it cannot be determined how far they extended from the body of the vertebrre, but they gradually subside as they pass backwards: the superior transverse processes are continued ·outwards from the sides of the posterior part of the body of the vertebra, and gradually subside as they advance forwards along three-fourths of the body of the vertebra : they are not continued into the anterior and inferior transverse processes, as in the Vicugna, but are separated therefrom by a narrow and shallow groove. The articular, or oblique processes, closely resemble those of the Auchenia FOSSIL MAMMALIA. 39 in form, and in the direction of the articular surfaces; those of the anterior processes 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, Pl. VII.) is given off immediately below the base of the anterior oblique process; this structure is not observable in any of the .cervical vertebrre of the Giraffe or Camelidre. In the form of the articulating surfaces of the bodies of the vertebrre the Macrauchenia deviates from the Giraffe and Camel, but resembles the Auchenire. In the Giraffe and Camel the anterior articulating surface is convex and almost hemispheric, the posterior surface is proportionally concave, so that the cervical vertebrre are articulated by ball and socket joints; yet not, as in most Reptiles, with intervening synovial cavities, but by the concentric ligamentous 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 vertebrre is much less than in the Camels; and in consequence they carry their necks more stiffly and more in a straight line. In Macrauchenia the anterior articulating surface (fig. 2, Pl. VII.) presents a still slighter convexity than in the Llama (fig. 4, Pl. VJI.), and the posterior surface (fig. I, Pl. VII.) presents a correspondingly shallower concavity. The form of the extremities of the body of the vertebrre, especially of the posterior, is sub-hexagonal, the breadth being to the depth as eight to five. The sides and under part of the vertebrre 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 Auchenire there is a longitudinal protuberance in the mesial line, instead of the two ridges. The two long cervical vertebrre 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 vertebrre 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 vertebrre on each other is more marked in the Camel, where the form and mode of articulation of the bodies of the vertebrre are designed to admit of a free and extensive inflection of the cervical vertebrre; and the result of this structure is very obvious in the sigmoid flexure of the neck in the living animal. In the Auchenire, on the contrary, the neck is can·ied less gracefully erect and in an almost straight line, and the form of the vertebrre and the nature of their joints correspond, as we have seen, to this condition. From the length of the bodies of the |