The Bill, or Beak, is composed of an upper jaw or maxilla, and an under jaw or mandible. From the figure it will be seen that "maxilla" is not strictly the whole upper portion, though the term is thus used for convenience, as is the plural "mandibles" for the two jaws when mentioned simultaneously. The "rhamphotheca," or horny sheath, may be simple (undivided), or compound, that is, made of several distinct pieces. In the _Anseres_ the covering is soft with a horny (corneous) tip or "nail"; in the _Limicolae_ it varies extremely, producing a hard pickaxe, as in the Oystercatcher, or a delicate sensory organ as in the Snipe and Woodcock. The rhamphotheca at times has extraordinary outgrowths, as in the Hornbills, Sheathbills, and elsewhere. In the _Accipitres_, or Diurnal Birds of Prey, and most _Psittaci_, the base is soft and becomes a "cere," while the similar formation in the _Columbae_ is due to a swelling of the _operculum_ or covering of the nostrils. This operculum, moreover, may be leathery (coriaceous), as in the _Charadriidae_, _Trochilidae_ and so forth, or rolled up, as in _Rhinochetus_; it may even result in a short soft tube, as in _Caprimulgus_, or in the hard double tube which gives the name of _Tubinares_ to the Petrels. "Impervious" nostrils are those with a septum, or division, between the nasal cavities, "pervious" {12}those with none. The narrow slit-like or entirely closed nostrils of the _Steganopodes_ should also be mentioned. The form of the bill varies from the "spoon" of _Platalea_ and _Eurynorhynchus_ (spatulate) to the "arch" of _Numenius_, the scissors of _Rhynchops_, the "wedge" of _Picus_, the big rounded feature of the _Psittaci_, and so forth; but for details the characters of the several Families must be consulted, as also for helmets, shields, horns, knobs, and peculiarities due to the elongation, distorting or crossing of the mandibles. These, too, are often notched, serrated, lobed or "festooned," or emarginate (slightly indented); the curious transverse serrations or _lamellae_ of the beak in _Anseres_, and the somewhat similar sifting apparatus in _Phoenicopterus_, _Prion_ and _Anastomus_ being especially remarkable. Teeth were probably lost by Birds before Tertiary times, but were possessed at least by _Archaeopteryx_, _Hesperornis_ and _Ichthyornis_. The so-called "egg-tooth" of embryos is merely a calcareous protuberance on the upper surface of the bill, which is cast after being used to crack the shell.