l6o ELEMENTS OE OENITHOLOGY. 



with our middle toe. It has ahnost always four joints, but may 

 have but three *- 



The fourth toe is comparable with the fourth toe of our foot — 

 a digit which, as analogous to the fourth digit of the hand, may, 

 at need, be distinguished as the annulus. It has nearly always 

 five joints, but may have four t or only three t. 



Our own hltle toe has no representative in the Class of 

 Birds. 



When there are but three digits, it is the liaUuz which dis- 

 appears, and when, as in the Ostrich, another is absent, it is the 

 second one. This rule is universal save in the genera Ceyx and 

 Alcyone, where the second digit is defective or hidden under the 

 skin, and the Babbler {Cholornis), where the fourth is defective. 



Disposition of the Digits. — Ordinarily birds have three toes 

 turned forwards and one, the hallux, turned backwards. Very 

 rarely, as in the Swift, all four are turned forwards, when the 

 hallux is the innermost of the four. Very often two toes are 

 turned forwards and two backwards, constituting what is termed 

 a zygodactyle or yoke-toed foot. In this case it is generally the 

 first and fourth digits which are turned backwards. In the 

 Trogons, however, the first and second toes are turned back- 

 wards and the third and fourth forwards. 



"When there are only three toes they all turn forwards, save 

 in the genus Picoides, where the fourth is turned backwards. 

 Sometimes a digit is what is called " reversible " or " versa- 

 tile " — that is, it can be turned either way, as is the case with 

 the fourth toe in most Owls. 



The hallux may vary much in its relation to the other toes. 

 In the first place, it may or may not be on the same level with 

 them — the 2nd, 3rd, and 4th toes always being placed side by 

 side at the bottom of the so-caUed " tarsus." If it is on the 

 same level with them, it is said to be incumbent. If it is 

 attached to the hinder side of the tarsus above the level of the 

 attachment of the other digits, it is termed elevated, as in the 

 Powl and the Duck. It is generally longer than one or two of 

 the other toes, and may be the longest digit of all. It is never 

 so long when elevated as when incumbent. It may barely 

 reach the ground, or it may, as in the Petrels, exist in a rudi- 

 mentary condition. 



* As in the Swift. t As in the Gi atsuoker. 



X As in the Swift. 



