ELEMENTARY SKETCH OF THE OSTEOLOGY OF BIRDS. 9 



occasionally absent. These bones on the inferior surface of the 

 skull will be again alluded to more fully below, as they are of 

 considerable importance in one of the modern schemes of classi- 

 fication of birds. By the moveable posterior articulations of the 

 palatines, sphenoidal rostrum, jugal arches, and quadrates, the 

 whole framework which supports the upper jaw is capable in 

 general of a certain extent of upward motion, which is especial- 

 ly marked in the Parrots. 



The posterior nares, or internal nostrils, are situated between 

 the palatines and the vomer. The Eustachian"^ tubes (or pas- 

 sages between the internal ear and the gullet or pharynx) gener- 

 ally pierce the basi-sphenoid, and open by a common aperture on 

 the inferior surface of this bone. (Plate 11. eu.) 



The jaws of all species of living birds are enclosed in a conti- 

 nuous horny sheath, with cutting edges and, no teeth ever 

 appear externally. In some foetal Parrots, however, teeth do 

 appear within the jaws, though they never cut the gum. 

 Certain extinct birds however [Odontopter^x^ and Icthyornis,X 

 were provided with a full series of teeth in both upper and 

 lower jaws, and in this character afford an almost complete 

 transition from the avian to the reptilian skull. 



General plan of the hones of the skull.— The student, who has 

 no previous acquaintance with osteology, may, perhaps, be 

 somewhat confused at this heterogeneous survey of the cranial 

 bones, and I have therefore added the following table, which 

 is intended to shew the general system on which the vertebrate 

 skull is constructed. This table is taken with some modifications 

 from one published by Professor Huxley. A few of the bones 

 named in this table were not mentioned in the bird-skull, where 

 they have no distinct representatives. 



It will be seen from this table that the brain-case is construct- 

 ed of three principal segments, which, reckoning from behind 

 forwards, may be called Occipital, Parietal and Frontal. Each 

 of these segments has a roof, lateral walls, and a floor, the 

 latter being composed of the Basi -occipital, Basi-sphenoid, and 

 Presphenoid, which conjointly with the applied Parasphenoid 

 (sphenoidal rostrum of the bird) are often collectively termed the 

 basi-cranial axis. The first and second segments are separat- 

 ed respectively by the ear and the eye, around the former 

 of which are collected a series of bones like the squamosal, 



* Eustachian, a term derived from tlie name of the discoverer of these passages. 



t Odontopteryx, (from ohovs a tooth, and opvii a bird) ; a fossil bird with teeth, 

 from the London clay. 



I Icthyornis, (from t'x^w? a fish, and opviQ a bird). A fossil bird from 

 cretaceous rocks of America, with teeth and with vertebrae hollowed on either side 

 (ampMcoelom) like those of fishes, whence the name is derived. 



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