20 BRITISH BIRDS 



form one of the most characteristic of the bones of the bird's 

 skeleton. In other animals the three bones are present, but they 

 are directed away from each other ; in the bird, as already described, 

 the pubis is directed backwards, parallel to the ischium ; in corre- 

 spondence, perhaps, with its position it has become a feeble bone, and 

 has but few muscles attached to it. The interest of the matter, 

 however, is mainly in the fact that among the extinct Dinosaurs, a 

 race of mesozoic reptiles, there were some in which the pelvis had 

 a very bird-like structure, with the same feeble and recurrent pubis. 

 This has been urged as a mark of affinity between the Dinosaurs 

 and birds. The several bones of the pelvis are free from each other 

 at the extremity, or almost so, in all the Ratites, and in the Tinamous, 

 which are supposed to bear some relationship to the Ratites. The 

 fact is interesting as being an example of the retention of a character 

 by one group of birds which is only transitional and embryonic in 

 another, for in all young birds the bones of the pelvis are separate ; 

 it is not until some time before hatching that they become fused 

 together as we see them in the adult. 



Hind Limb. 



At first sight there appears to be a considerable difference between 

 the fore limb and the hind limb. In both there is a long proximal 

 bone, called humerus in the one case and femur in the other, followed 

 by a pair of bones the tibia and fibula corresponding to the radius 

 and ulna of the fore limb. But in the hind limb (fig. 13), the foot 

 proper, consisting of metatarsals and phalanges, appears to come 

 immediately after the tibia and fibula. In a sufficiently young bird, 

 what is the apparent lower end of the tibia, and what is equally 

 apparently the upper end of the metatarsus, are detachable ; these 

 two halves which are thus detachable are the tarsus, which is the 

 equivalent of the carpus of the wing. The lower bone of the leg is 

 on this account usually spoken of as the tarso -metatarsus. The 

 lower part of this bone is made up of three fused elements, the separa- 

 tion of which from each other is clearly apparent at the lower end 

 of the bone, where the phalanges are attached. In the Penguins 

 the three bones are separated by grooves of a very marked character 

 throughout. In some birds there is a fourth toe, the hallux ; in these 

 cases there is a small separate metatarsal loosely fixed to the lower 

 end of the large conjoint metatarsals. 



