I O SKELETON CHAP. 



elements unite with the metatarsus, which is therefore properly 

 a " tarso-metatarsus/' though often called merely " tarsus " by 

 ornithologists. This arises from a fusion of the second, third, 

 and fourth metatarsal bones, which in the adult (except among 

 the Sphenisci and to some extent in Psittaci) do not lie in the 

 same plane ; the middle one having its upper end thrust back- 

 ward and its lower end forward in the course of growth to 

 maturity. The fifth metatarsal practically disappears, while the 

 first remains more or less separate, and lies behind the distal 

 portion of the other metatarsals. 



Of the toes the fifth is not traceable in Birds ; the first is 

 often aborted, but the second only in Struthio, and to a less 

 extent in Ceyx and Alcyone, and the fourth (nearly) in Cholornis. 

 The hallux, or hind toe, has two phalanges, the second digit 

 three, the third four, and the fourth five ; Cypselus and Panyptila 

 (Swifts), however, are exceptions, and possess only three in each 

 of the anterior toes, while the Caprimulginae (true Nightjars) and 

 Pteroclidae (Sand-Grouse) have only four joints on the outer. In 

 Owls and Pandion the fourth digit is reversible at will, the same 

 being true to a less extent of the Musophagidae (Plantain-eaters) 

 and Leptosoma (akin to the Koller) ; when this condition is per- 

 manent, as in the Cuculidae, Psittaci and Pici the foot is termed 

 zygodactylous. In Trogones the second toe is reversed (hetero- 

 dactylous). Colius can turn the first toe forward and the fourth 

 backward, while certain Swifts, and to a less degree some Night- 

 jars, have the whole number permanently pointing to the front 

 (pamprodactylous). Membranes more or less connecting the 

 anterior digits produce a webbed or swimming foot, even the 

 hallux being united with the rest in the Steganopodes. The 

 hind-toe is often elevated, or higher than its fellows, when it is 

 commonly reduced and sometimes lacks a nail. The Ostrich has 

 little or no claw on the outer toe, while that of the third toe is 

 toothed or serrated in a considerable number of Birds, but this 

 is a character of very slight importance. 



The covering of the metatarsus is usually " scutellated," but 

 when the scutellae, or scales, which may be oblong or polygonal, 

 are smaller than usual and generally hexagonal it is called 

 " reticulated." In some cases the surface becomes nearly or quite 

 smooth (" ocreated " or " booted "), or more or less granulated. 



8. The structure of the Skull is a study in itself and affords 



