3o6 THE THIGH AND LEG. [chap. 



the Armadillos and Orycteropus, and a sharp ridge along 

 the whole external border in Myrmecophaga. The fibula is 

 as long as the tibia. In the Armadillos these bones are 

 commonly ankylosed together at each extremity, but curve 

 away from each other at the middle, leaving a wide inter- 

 osseous space. In the Anteaters they are both nearly 

 straight and parallel. 



In the Sloths the femur is long, slender, and flattened 

 from before backwards. There is no third trochanter ; the 

 head is large and globular, and placed near the middle of 

 the proximal end of the shaft, with the axis of which it 

 more nearly coincides than in most Mammals. The tibia 

 and fibula are complete, and more nearly equal in size than 

 in most ^Mammals. They are both curved, so as to be 

 separated considerably in the middle part of the leg. The 

 lower end of the fibula has a conical prominence which 

 turns inwards, and fits into a depression on the outer side 

 of the articular surface of the astragalus, as a pivot into 

 a socket. 



In none of the Marsupialia is a third trochanter pre- 

 sent on the femur. The fibula is always well developed, 

 and its upper extremity is often produced into a well-marked 

 process, to the top of which a sesamoid bone is not un- 

 frequently attached ; but, on the other hand, the patella, 

 except in the Peramelidce, is unossified or quite rudimentary. 

 In the climbing Australian Phalangers and Koalas, which 

 have broad hind feet, with an opposable hallux, there is a 

 greater freedom of movement between the fibula and tibia 

 than in other Mammals, approaching in some degree to the 

 rotation often permitted between the radius and ulna. 



In the MoNOTREMATA the femur (Fig. 113,/) is of very 

 remarkable form, being short, flattened from before back- 

 wards, narrow in the middle of the shaft, and very broad 



