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Show 1896.] ANATOMY OF PETROGALE XANTHOPUS. 689 allows pronation and supination to the extent of rather less than a quarter of a circle. The Interosseous membrane is present between the lower two-thirds of the bones; it is strong and has the normal direction. The Inferior radlo-ulnar joint has no synovial cavity, and there is no triangular fibro-cartilage. The Wrist-joint consists of two separate synovial cavities ; the first between the radius and scapho-lunar is normal, in the second the peg-shaped lower end of the ulna fits into a deep concavity in the cuneiform. The Sacro-iliac joint consists of a crescentic articular surface on the sacrum and ilium, with the concavity forward, I. e. towards the head. In front of this articular surface the bones are very rough for a considerable area and are bound together by very powerful sacro-iliac ligaments. There is a distinct synovial cavity between the cartilage-covered surfaces, so that the joint cannot be described as a synchondrosis. Considerable gliding movement is allowed by which the crescentic surface of the ilium describes a small segment of a circle over the similar surface of the sacrum, tbe centre of the circle being about the middle of the sacro-iliac ligament, the fibres of which are lax enough to allow a certain amount of play. The mobility of this joint is no doubt connected with the great size of the psoas parvus ventrally and the erector spinse dorsally, aud is an adaptation to the Kangaroo's mode of progression. Before taking its leap the animal probably flexes the pelvis on the sacrum by means of the psoas parvus, after which it suddenly straightens its back and extends the pelvis by means of the powerful erector spinas, in this way assisting the leg-muscles in taking the spring. It is worth mentioning that the left sacro-iliac joint of the animal I dissected was affected with extensive tubercular disease ; M r . Bland Sutton tells me that he has met with this condition on more than one occasion, and considers that it is connected with the mobility of the joint. The Pubic symphysis is formed by a strong cartilage which connects the pubic bones of opposite sides and has no synovial cavity. At the posterior part of the joint the cartilage divides like an inverted Y to include the triangular subpubic bone. In the Hip-joint the capsule is attached above to the margin of the great trochanter, the whole upper margin of the neck of the femur being covered by articular cartilage. Below it is attached to the margin of the head and neck. Anteriorly much more of the neck is included in the capsule than posteriorly. The iliofemoral band is present, but the thickest part of the capsule is the upper and back, which is doubtless an adaptation to the usual position of the joint in the Kangaroo, a position of extreme flexion and external rotation. The cotyloid ligament is much thicker posteriorly than elsewhere ; it is continued into a triangular transverse ligament across the very deep cotyloid notch. The liga- |