iK@) 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 he 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 les 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 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 Roller); when this condition is permanent, as in the 
Cuculidae, Psittaci and Pici the foot is termed zygodactylous. 
In Zrogones the second toe is reversed (heterodactylous).  Colius 
can turn the first toe forward and the fourth backward, while 
certain Swifts, and to a less degree some Nightjars, 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 some- 
times 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 
“yeticulated.” 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 
