OCR Text |
Show 84 ZOOLOGY OF THE VOYAGE OF TilE BEAGLE. VERTEBRAL CoLUMN. fthe Scelidothere Mr Darwiu's specimen includes, Of this part o~ the skeleton o I ervical art ~f th~ dorsal, and the sacral series as is represented m Plate XX., t 1e c . .' P of vertebrre in a more or less perfect comhtwn. . . The cervi. cal verte b rre prescn t the ordinary mam. malian number, seven, and .a t'he free or o ar t1. cu 1a t e d as to have permitted reciprocal movecm enth upon t e1a .t..: 1 othe'r . The·u transverse proce ses ac re perforated as usual 1• 0r t e ver edu tla arten·e s 1, hese p1.0 cesses m· tl1 e atlas are remarkable for theu great brea kt ld, 1 th ~nd thickness. and indicate the muscular forces which must h~ve wo~· e t~t:ghe~d upon the spit~e to have been very powerful. The axis is provided wtth a robust' processus dentatus,' having a base equal in ?readth to the body. of the axi it elf; and a smooth articular convexity on the stde of. the ap.ex on whtch. tl~e ring of the atlas rotated. The line of union between the axis ~nd Its cl~ai:actenstt proce , which here resembles the body of an abortive verte~Ha, IS very d1stmc~ . Th transver ~ e processes of the vertebra dentata are comparatively feebl~, but tlus condition i amply compensated for by the great development of the ~pmous proce s. (Pl. XXIV. fig. I.) This process is bent backwar~s at ue~rly a nght angle, overlaps with its reflected extremity the spine of the thn·d .cerviC~l vertebra, and rests by its ba e, on the under part of which are the posteno~ articular surfaces, upon the broad and strong anterior oblique processes of the thu·d vertebra. The third, fourth, fifth, and sixth cervical vertebrre have moderately developeu and pointed pinou proce ses: theit· transverse proces e are broad, and extend obliquely backward , and slightly overlap each other. On the under part of the transverse process of the sixth cervical vertebra there is the fractured base of what I conjecture to have been an expanded aliform plate, analogous to that observable in the corre ponding vertebra of the Orycterope. The seventh cervical vertebra has part of the articular de pres, ion for the head of the first rib upon each side of its body: the transverse process is feebly developed, but the spine is double the height and size of those of the preceding vertebr~. The spinous process of the fir t dorsal vertebra in like manner rises to twice the heiD'ht of the preceding spine of the seventh cervical, and preserves an equal anteroposterior diameter from its ba e to its summit, which is thick and slightly bent backwards: four or five succeeding dorsal vertebrre give evidence of having been surmounted by spines of equal height and strength. The transverse proce ses of these dor al vertebr~ present bold concavities ou their inferior part for the reception of the tubercles of the rib , and they gradually ascend upon the base of the pines a the vertebrre are placed further back, so as to increase the expansiveness FOSSIL lllAMMALlA. U5 of the chest. The state of the fossil did not afford further information as to tht:: condition of this part of the vertebral column, but the parts which have been pt·eserved are precisely those from which the most interesting inferences as to the affinities and habits of the extinct quadruped can be deduced. Whether the Megatherium be most nearly allied to the tribes of the Sloth or Armadillo has been a question under recent discussion, and, as a corollary of this problem, whether its habits were those of a scansorial or of a fossorial quadruped. For, strange as it may appear at first sight, there have not been wanting arguments, and those Ul'ged by an anatomist to whom we owe much novel and intet·esting information respecting the extinct Edentata, in support of the belief that the Megatherium, gigantic and ponderous as must have been its frame, actually climbed trees like a Sloth, and had claws and feet organised for prehensile actions, and not in accordance with that type by which they arc usually adapted for digging up the soil.* Now, in whatever degree the Megatherium may be involved in this question, the smaller Megatherioid species at present under consideration must be at least equally implicated in it. In the adaptation of the frame of a mnmmiferous quadruped for especial and peculiar actions and modes of life, such as for climbing and living in trees, or for burrowing and seeking concealment in the earth, not only the immediate instruments, as the feet, are modified, but the whole of the osseous and muscular fabric is more or less impressed with corresponding adaptations, whilst at the same time these special adjustments are invariably subordinated to the type of organization which characterizes the group. The type of the order B1·uta or Edentala is well-marked; one or more claws of unusual length and strength, characterize the fore-feet and sometimes the hindfeet in evet·y genus, and the term 'Macronykia' would more aptly designate them than the term which Cuvier substituted for the good old Linnrean appellation. The uniform absence of true roots to the teeth, where these are present, is another general character; the skeleton exhibits many well-marked peculiarities common to the whole ordet·; while at the same time it is modified in various modes and deO0 'rees in accordance with the peculiar habits and exigencies of the species. One of the regions of the skeleton which manifests adaptive modifications of this kind in the most remarkable degree is the cervical division of the vertebral column. In one edentate species it is lengthened out by two additional vertebrre more than in any other mammal; in another it is reduced by anchylosis to as great an extent below the regular number of moveable pieces: and these, the two most opposite conditions of the cei·vical vertebrre which are to be met with in the mammiferous class are related to equally diverse and opposite habits of life. • Lund, Vidcnskabcrnes Selsknbs, Natur: og 1\Iathem. Afhandlinger, Kiobcuhavn, vo!. viii. |