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 _Trogones_ 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 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 {11}considerable help in Taxonomy (Classification). It must suffice here to refer for the names of the parts to the subjoined figure. [Illustration: FIG. 5.–Skull of a Wild Duck (_Anas boscas_), from the side. _ag_, Angular; _als_, alisphenoid; _ar_, articular; _bt_, basitemporal; _d_, dentary; _en_, external nostrils; _e.o_, exoccipital; _eth_, ethmoid; _fr_, frontal; _j_, jugal; _lc_, lacrymal; _mx_, maxilla; _mx.p_, maxillopalatine process; _n_, nasal; _p_, parietal; _pg_, pterygoid; _pl_, palatine; _ps_, presphenoid; _px_, premaxilla; _q_, quadrate; _q.j_, quadratojugal; _s.ag_, supra-angular; _s.o_, supraoccipital; _sq_, squamosal: _ty_, tympanic cavity; _v_, vomer; _II_, foramen for optic nerve; _V_, for trigeminal. (From Wiedersheim, after Parker.)]