1896.] ANATOMY OF PBTROGALE 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 radio-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, the 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 spinae . dorsally, and 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 

 psbas parvus, after which it suddenly straightens its back and 

 extends the pelvis by means of the powerful erector spinsB, 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 ; Mr. 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 Puhic 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 ilio- 

 femoral 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 trans- 

 verse ligament across the very deep cotyloid notch. The liga- 



