342 THE VERTEBRATE SKELETON. 



metatarsals, forming a compound bone, the tarso-metatarsus. 

 The first metatarsal is nearly always free but occasionally 

 as in Phaethon it is fused with the others. No adult bird 

 has more than four digits in the pes. In the Penguins the 

 metatarsals are separate, and in many birds larger or smaller 

 gaps exist between the fused metatarsals. In most birds 

 the third metatarsal is curved so as not to lie in the same 

 plane as the others, but in the Penguins they all three lie in 

 the same plane. The metatarsals are clearly separated in 

 Archaeopteryx. In Gallinaceous birds the tarso-metatarsus 

 bears a bony outgrowth which is sheathed in horn and forms 

 a spur. 



In most birds the first four toes are present while the fifth 

 is always absent. The first toe commonly has two phalanges, 

 the second three, the third four, and the fourth five. In 

 Swifts the third and fourth toes have only three phalanges. 

 Many birds, such as all Ratitae except Apteryx, have only three 

 toes, the hallux being absent ; in the Ostrich the second toe is 

 also gone with the exception of a small metatarsal, so that the 

 foot retains only the third and fourth digits, the third being 

 much the larger of the two and bearing a claw, while the 

 fourth is clawless. 



In the Swifts, Cormorants, and Penguins, all four toes are 

 directed forwards. In most birds the hallux is directed back- 

 wards, and the other toes forwards. In the Owls the fourth 

 toe can be directed backwards as well as the hallux, while 

 in Parrots, Cuckoos, Woodpeckers, and Toucans the fourth toe 

 is permanently reversed. In Trogons the second toe is reversed 

 in addition to the hallux, but not the fourth. 



