2 STRUCTURE AND CLASSIFICATION OF BIRDS 



as shown by FORBES, a rudiment of the missing toe is 

 present in the shape of a nodule, or of two nodules, of bone, 

 hidden in extreme cases beneath the skin, and only appear- 

 ing externally as a small wart with no claw. In Phebcetria 

 there is an advance upon this, since there is externally a 

 minute claw, and beneath the skin two minute nodules of 

 bone. In the woodpeckers, Picoides and Tiga, commonly 

 spoken of as three-toed birds, there is a similar vestige of 

 the fourth toe. On the other hand this does not apply to 

 all three-toed birds. In Rkea, Tetrax, and Pelecanoides 

 FORBES searched in vain for a trace of the missing toe. The 

 toes of the bird's foot are arranged in different fashions, 

 giving rise to more than one form of foot. When there are 

 only three toes they are all directed forwards, except in 

 Picoides, where the last is directed backwards ; but then, as 

 already stated, this bird has a rudimentary hallux, and it 

 conforms therefore to the type seen in other woodpeckers. 

 AVhen there are four toes they are rarely all turned forwards ; 

 this is the case, however,' with the swifts. Most commonly 

 the hallux is turned more or less completely backwards ; 

 this is so with the passerines and with many other birds. 

 In what is termed the zygodactyle foot, e.g. the woodpeckers, 

 both the first and the fourth toes are turned backwards, and 

 thus an effective grasping organ is produced. 



An anomalous form of zygodactylism, termed hetero- 

 dactylism by some, is offered by the trogons, where the 

 second toe is turned back. Syndactylism is an expression 

 used to describe toes which are united together for a longer 

 or shorter distance, such as, for example, the todies and 

 kingfishers. Further details in the form of foot will be 

 found under the descriptions of the several families. 



In many birds the toes are perfectly free from each other 

 up to their attachment to the metatarsals. In others there 

 is a condition known as webbing, where a scale-covered 

 skin is stretched between the toes. This may be feebly 

 developed, as in many wading birds, or complete, as in swim- 

 ming birds, such as the duck. The extreme state of webbing 

 is seen in the pelican tribe, where all the four toes are 



