cc() STRUCTURE OF VERTEBRATES 



the fusion of tibia and fibula and radius and ulna. Except where, 

 as in the forearm of man, they can be rotated on one another, 

 there is presumably no advantage in having two bones in this 



position. 



The reptiles show a wide range of limb structure, including 

 types which are primitive and others which are highly specialised 

 (Figs. 433, 434). The snakes have no limbs at all, but a few have 

 small, functionless femurs which do not project beyond the body- 

 wall. Limbless lizards, such as the slow- worm {Anguis fragilis) of 



TABLE VI. 



Nomenclature of bones of wrist and ankle, preaxial border on left. The names 

 recommended for zoological use are in the top line in each row ; where the names 

 recommended by the Anatomical Society of Great Britain differ from these, they 

 are in the second line. Names in the third line are best avoided. 



Britain, are also without limb skeletons. The structure of the wing 

 of a bird (Figs. 315, 320) is primitive in that both ulna and radius 

 are separate bones and unspecialised in form, but the wrist and 

 hand are much modified. The reduction of the digits is presumably 

 connected with flight, as only one finger is needed to support 

 the main feathers, but the peculiar fusion of the metacarpals 

 with the distal carpals seems to be merely a peculiarity of no 

 functional importance. It is paralleled in the hind-limb (Fig. 320), 

 and was probably found also in the bipedal dinosaurs. 



The limbs of mammals show an extraordinary diversity, and 

 illustrate better than almost any other structure the principle of 

 adaptive radiation by which one basic plan becomes modified to 

 fit different habits and habitats. A primitive type is found in a 



