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Show 1901.] ANATOMY OF CHRYSOCHLORIS TREVELYANI. 33 away from the rudimentary coracoid, and all ligamentous connections between the two are obliterated. To the comparative anatomist the contrast between the arrangement met with in this animal and the Three-toed Sloth (Bradypus) is very striking. In the latter animal the coraco-clavicular ligament forms the only union between the clavicle and the scapula, while in the Golden Mole it is absent altogether. The Armadillo, as will be seen on referring to m y paper1, forms a transition between the two. The acromio-clavicular articulation is very interesting. A small bone is interposed between the acromial end of the clavicle and the acromion, and there is a synovial cavity on each side of it; this is the only instance I have hitherto noticed of the presence of such a bone, and the possibility of its being homologous with the interarticular meniscus of the same joint in man at once occurs to the mind. Personally, I should be more inclined to regard the two structures as analogous than homologous, because intraarticular menisci are so often found where rotation is combined with hinge or gliding movements; and in Chrysocldoris, partly owing to the absence of the coraco-clavicular ligaments, the rotation or pendulum movement of the scapula is very free in addition to gliding movement. Probably this little bone is an ossified meniscus developed to meet the requirements of the joint. Of course this is merely a surmise, but it seems borne out by the facts at m y disposal. The great differences in the detail of the bones and joints connecting the shoulder-girdle with the trunk in the Mole (tcdpa) and the Golden Mole are very obvious. The shoulder-joint differs little from that of a generalized mammal, it is chiefly remarkable for the lateral compression of the head of the humerus. The oblique middle gleno-humeral ligament is well marked, but does not project at all into the cavity of the joint; it has the typical mammalian attachments. It becomes tight in extension and external rotation. The elbow is chiefly remarkable for the enormous development of the olecranon, but there is no upward extension of the back of the head of the radius as in the Mole. During flexion of the joint considerable lateral movement is allowed, as well as rotation of the radius and ulna together, through an axis passing between the two bon?s and parallel to their long axes ; this movement is possible to the extent of about | of a circle, and is quite distinct from pronation of the radius, which is only possible for about ^ of a circle. As there is so little true pronation one would not expect an orbicular ligament, nor can anything of the land be found. The external lateral ligament runs to the outer side of the head of the radius, v bile the internal is fan-shaped and passes down to the inner margin of the sigmoid cavity of the ulna. The wrist-joint resembles that of most mammals in which there is little pronation and supination of the forearm. There is no 1 Op. cit. p. 50. PROC. ZOOL. Soc-1901, VOL. I. No. III. 3 |
