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Show 88 ZOOLOGY OF TilE VOYAGE OF TilE BEAGLE. h · was used in nid . . g therefore no such mec amsm, for its own mere support requmn .. of the fossorial actions of the extreroittes. h . have their processes compara- . I t 1 rre of the Megat enum As the cervlCa. ver e l . . the anterior dorsal spines are rela-tively weaker than m the Scelldotherlmm, lantd ere the extent or nature ofthe I 't b concluded t mt w 1a ever w tively s 1ortcr, 1 may e · ' . h 1. 1 't vas provided, the head did not fo soriallabours of the enormous claws Wit w 11C 1 l ' . . de ree co-operate with the diggingOirnplements i~tt!~~~~:~:e~i~~Je t~~~.;nistl:: ~::~fica~ion as in the Scclidothere and rycterope. '" · 'th those which of the cervical reaion of the spine of the Megathere correspondm~ wt k I . h we have seen to be subservient to the arboreal habits of the slot ' a rembar wt~IC " b 1 h h ve perused the acute o serva wns will not be deemed superfluous y t 1ose w o a . . . .ld d b M Lund in favour of the scansol'lal charactei of the and arguments au uce y · extremities of the Megatherium and Megalonyx. . . . The fragments of the dorsal vertebrre and r~bs of the Sce~1doth~num, ~h1~h are figured in Plate XX, offer no modifications wh1ch need detam our. attentwn, ~hey closely conform, excepting in the greater relative height of the anter~or dor~al ~pmcs, already noticed, with the Megatherioid type .. The sacrum m~tmfests m .~t~ vast expanse, the great development of th.e poster10.r transverse p~oc~sses to ,.1om th~ ischium, the capacious medullary cav1ty, and w1de nervous f01m~ma, a ~Jke con formity with the Megatherium, and a corresponding harmony w1th the dispropor-tionate bulk of the hind legs. Bo ES OF THE ExTREMITIES. The Scapula in its double spine, the osseous arch formed by the confluence of the acromion with the coracoid process, and the substitution of a distinct foramen for the suprascapular notch, agrees with that of the Megatherium : but the span of the acromial arch is relatively wider, and the surface for the articulation of the clavicle is better marked. This articular surface, which is distinctly shewn upon the acromion of both the scapulre in Pl. XX. is the more interesting, as being the only evidence of the clavicle of the Scelidothere which we at present possess; but it is enongh to prove that this quadruped enjoyed all the advantages in the ac:tions of the fot·e-extremity, which arise out of the additional fixation of the shoulder-joint afforded by the clavicle-a bone which the extinct Megatherioids are the largest of the mammiferous class to possess in a completely developed state. The form, position, and aspect of the glenoid cavity tor the humeru~ closely correspond with the condition of the same part in the Megatherium. The limits of the acromial and coronoid portions of the arch were still defineable in the FOSSIL MAMMALIA. 89 pres~n_t skeleton, which indicates the nonage of the individual in the unanchylosed cond1tton of most of the epiphyses. In regard to the presence of a clavicle in tbe Megalonyx M. Lund has deduced certain co?clusion~, which, if well founded, would be equally applicable to the present allied species, and to the great Megatherium. I am induced, therefore, to offer a few physiological observations on that bone, which appear to me to l.eatl to a ~ore correct interpretation of its uses and relations in the great mam1mferous ammals now under consideration. When ~he ant~rior extremities in mammalia are used simply for the purpose of progressive motwn on dry land, as in the Pachyderms and Ruminants or in water, as in the Cetaceans, there is no clavicle; this bone is introduced b~tween ~h~ sternum and acromion, in order to give firmness and fixity to the shoulderJOil1t when the fore-leg is to discharge some other office than that of locomotion. In these cases.. howe.ver, the clavicle exists in various degrees of development, and even Its rudiment may be dispensed with in some of the actions which require a considerable extent of lateral or outward motion, and of freedom of ~otation of the fore-limb. When, therefore, we find the clavicle fully developed m the skele~on o~ an extinct mammiferous animal, and so placed as to give the humeral artiCulatiOn all the benefit of this additional mechanism, we may confidently expect that it will afford an insight into the habits and mode of life of such ~xti~ct spec~es. M. Lund* has argued from the clavicle of the Megalonyx, that 1t. clunbcd like a Sloth. "Animals," says Sir C. Bell,t "which fly or dig, 01· chmb, as Bats, Moles, Porcupines, Squirrels, Ant-eaters, Armadilloes, and Sloths, have this bone; for in them, a lateral or outward motion is required." But in regard. to tl~e present problem, we have to enquire whether the clavicle manifests any modtficatwns of form, of strength, or development in relation to the special differences of these several actions, with which its presence is asserted to be associated? . In mammals which fly, the clavicle is always complete: the rabbit, the fox, and the badger are instances of burrowing animals in which the clavicle is absent or rudimental. The presence of a perfect clavicle is not more constant in climbing quadrupeds. The Ai, for example, has an incomplete clavicle, which is attached to the acromion process, and terminates in a point about one-fourth of the distance between the acromion and the top of the sternum, to which the cia;icular style is attached by a long slender ligament: the advantage, therefore, wluch a perfect clavicle affords in the fixation of the shoulder-joint, is lost to this climber pa1· excellence. Again, the Bears, which arc the bulkiest quadrupeds that are gifted with the faculty of climbing, and this in so perfect a degree that the Sun-bears of the Eastern Tropics may be termed arboreal animals,-these scan- • Loc. cit. t Bridgewater Treatise, p. 46. N |