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Show 36 ZOOLOGY OF TIIE VOYAGE OF THE BEAGLE. nearly entire, the proximal and distal extremities of the left tibia and fibula ; and a metatarsal bone of the left hind foot. . Before entering upon the description of these remains, a few observa~wns may be advantageously premised on some of the distinguishing. cha~acter~ of th.e Camelidre. It is well known that the Camels and Llamas devmte m their dentition, viz., in the presence of two incisors in the upper jaw, from th~ tru.e Ru~inants; and we cannot avoid perceiving that in this particular the d.irectl~n ~~ w~1ch they deviate tends towards the conterminous Ungulate Order, m wluch InCisor teeth are rarely absent in the upper jaw. They also further deviate from the Ruminants and approach the Pachyderms in the absence of cotyledons in t~e ~terns. and fetal membranes; having, instead thereof, a diffused vascular vtllostty of the chorion, as in the sow and mare. But besides these characters, by which, in receding from one type of hoofed mammalia, the Camelidre claim affinity with another, there are many parts of their organization peculiar to themselves ; of some of these peculiarities, the relation to the circumstances under which the animal exists, can be satisfactorily traced; in others, the connection of the structure with the exigencies of the species, is by no means obvious, and in this predicament stands the osteological peculiarity, which is immediately connected with our present subject-a peculiarity in which the Camelidre differ not only from the other Ruminants, but from all other existing Mammalia, and which consists in the absence of perforations for the vertebral arteries in the transverse processes of the cervical vertebrre, the altas excepted. I may observe that what is described as a perforation of a single transverse process in a cervical vertebra is essentially a space intervening between two transverse processes, a rudimental rib, and the body of tl1e vertebra. In the coldblooded Sau~·ians,-in which the confluence of the separate elements of a vertebra takes place tardily and imperfectly, if at all,-the nature of the so called perforation of the transverse process is very clearly manifested, as in the cervical vertebrre of the Crocodile, in which the interspace of the inferior and superior transverse processes is closed externally by a separate short moveable cervical rib. In the Ornithorhynchus paradoxus the vertebra dentata also preserves through-. out life this condition of its lateral appendages : in other Mammalia it is only in the fcetal state that the two transverse processes are manifested on each side with their extremities united by a distinct cartilage, which afterwards becomes ossified and anchylosed to them. In the Hippopotamus the inferior transverse process sends downwards a broad flat plate extended nearly in the axis of the neck, but so obliquely, that the posterior margins of these processes, in one vertebra, overlap the anterior ones of the succeeding vertebra below, like the cervical ribs in the Crocodile; the same structure obtains in many other mammalia, especially in the Marsupials. In the FOSSIL MAMMALIA. 37 Giraffe, the inferior transverse processes are represented by relatively smaller compressed laminre, projecting obliquely downwards and outwards from the anterior and inferior extremity of the body of the vertebra. The superior transverse processes in this animal are very slightly developed in any of the cervical vertebrre, and the perforation for the vertebral artery is above and generally in front of the rudiment of this process, being continued as it were through the side of the substance of the body of the vertebrre. In the long cervical vertebrre of the Camel and Llama, the upper and lower transverse processes are not developed in the same perpendicular plane on the sides of the vertebrre, but at some distance from each other ; the lower transverse processes (a, fig. 1, Pl. VI.; a, fig. 1, 3, 4, Pl. VII.) being given off from the lower part of the anterior extremity of the body of the vertebra; the upper ones (b, fig. 1, Pl.. VI.; a, fig. I, 3, 4, Pl. VII.) from the base of the superior arch near the postenor part of the vertebra, or from the sides of the posterior part of the body of the vertebrre. The extremities of these transverse processes do not become united t?gether, but they either pass into each other at their base, or continue throughout hfe separated by an oblique groove (as in fig. I, Pl. VI.) This groove would not, however, a_fford s.ufficient defence for the important arteries supplying those parts of the bram whteh are most essential to life; and, accordingly the vertebral arteries here deviate from their usual course, in order that adequate protection may be afforded to them in their course along the neck. From the sixth to the second cervical vertebrre inclusive in the Auc!tenim, and from the fifth to the second inclusive in the Cameli, * the vertebral arteries enter the vertebral canal itself: alon()' with the spinal chord, at the posterior apertme in each vertebra, run forward~ on the outside of the dura mater of the chord between it and the vertebral arch, and when they ~ave thus traversed about two-thirds of the spinal canal, they perforate respectively the superior vertebral laminre, and emerge directly beneath t~e anteri01: oblique or .articulating processes, whence they are continued along wtth the spmal chord mto the vertebral canal of the succeeding vertebra, and perforate the sides of the anterior part of the superior arch in like manner; and so on through all the cervical vertebrre until they reach the atlas, in which th ir disposition, and consequently the structure of the arterial canals, resemble those in othet· Ruminants. The two cervical vertebrre of the Macrauchenia present precisely the struc- * In the seventh cervical vertebra of the Camel, as in many other Mammalia, there is no perforation in any part for the vertebral arteries. In a Vicugna, I find the same structure; but in a Llama, the side of tho body of tho seventh cervical vertebra is p rforated longitudinally on tho right side. In the Camel, the vertebra. l arteries pierce ~ho sixth cervical vertebm, immediately below tho superior transverse proces es, and pass obhqucly to the antcr~or aperture of the cervical canal, where they cmorge beneath the anterior oblique procesoe , and then enter the spmal canal of the fifth cervical vertebra, as do cribcd in the text. |