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Show 1882.] PROF. WT. H. FLOWER ON THE EDENTATA. 361 In tracing true affinities we must not look to these obviously adaptive characters, but toothers of really deeper significance. To commence with the skeleton, one of the most striking characters by which the Myrmecophagida differ from ordinary mammals is the presence on the posterior dorsal and the lumbar vertebrae of accessory articulating surfaces, in addition to the true zygapophyses common to all mammals, and causing a remarkable interlocking of the arches of these vertebrae. Such articulations are found equally well-developed in the Megatheriida and in the Dasypodida, and in a comparatively rudimentary state in the Bradypodida, especially in Hradypus 1. The whole vertebral column of the Sloths, especially the dorsal and lumbar region, is poorly developed, evidently in relation to the subordinate function of the muscles attached to it. In the ordinary position the animal hangs below the branches of the trees in which it dwells, the trunk being merely slung between the two pairs of extremities ; progression is effected chiefly by the body being dragged along by the fore limbs ; the hind legs, which usually take so active a part in supporting and propelling the trunk, are small and weak, andthe functions of walking, running, andjumping are alike in abeyance. Hence all the processes of tbe vertebral column-the spinous, transverse, and accessory-are but feebly developed. The existence then, even in a rudimentary form, of these additional articulations is extremely significant. It may almost be said that they prove that the Sloths are descended from animals in which they existed in a fully developed form. On the other hand, like as in some respects the vertebral column of Manis is to that of Myrmecophaga, not a trace of either of these articulations or of the processes on which they are situated are to be found in the former. There is a strong; inter-locking of the lumbar vertebrae ; but it is formed by the greatly curved form of the true zygapophyses, and not by the addition of any superadded parts. In Orycleropus also they are entirely absent. On this ground alone we might be justified in assuming that the Old-World Edentates are not closely related to the American forms. Nothing can be more different than the characters of the sternum of Myimecophaga and of Manis. In the former the numerous mesosternal segments are small, laterally compressed, and articulated with each other and with the strongly ossified sternal ribs by synovial joints. In Manis the sternum is broad and flat, and the sternal ribs only partly ossified, and connected with it in the normal maimer. In these characters, the Bradypodida, Megatheriida, and Dasypodida agree with the Anteaters, and Orycleropus is more nearly related to Manis. The gigantic post-Tertiary Edentates of the New World, Megatherium and its allies, throw* much light upon the close affinity of the Sloths and true Anteaters. By common consent they have been placed among the former when the order is divided into two divisions, and are spoken of as "Ground-Sloths;" yet in many important characters, perhaps in all those not relating to the functions of 1 See ' Osteology of Mammalia,' 2nd edit., 1876, p. 55. |