GENERAL STRUCTURE. 121 I 



passage from the mouth. In this respect, therefore, the develop- 

 ment of the Bird's skull does not attain such a specialisation as that 

 of the existing Crocodiles. The quadrate is usually movably at- 

 tached to the squamosal ; and the articulation of the palatopterygoid 

 bar to the basipterygoid processes of the sphenoidal rostrum is also 

 a movable one, by which means the premaxillary beak can be moved 

 to a certain extent upon the rest of the skull. The vomers are 

 subject to great variation. They underlie the ethmosphenoidal 

 region, and when present are connected posteriorly with the pala- 

 tines, except in the Ostrich. The relations of these and the other 

 bones of the palate form important features in Professor Huxley's 

 classification of Birds ; but since this is a subject to which the 

 attention of the Palaeontologist is but seldom directed, the reader 

 desirous of further information must refer to other works. The 

 jugal and the quadratojugal are slender, rod-like bones, of which 

 the former articulates with the equally slender maxilla, and the latter 

 by a hollow surface with the quadrate. In all existing Birds the 

 dentary elements of the two rami of the mandible are always found 

 welded at the symphysis into a single bone ; but in the Cretaceous 

 Ichthyornis, and perhaps in other Mesozoic forms, this union is 

 imperfect. There is frequently a lateral vacuity between the den- 

 tary and splenial, like that of the Crocodilia. The angle of the 

 mandible may be either truncated, or produced into a long recurved 

 process, as in the Fowls (fig. 1103), Ducks, and Geese. In exist- 

 ing and Tertiary Birds the beak is ensheathed in horn, and is 

 totally devoid of teeth ; but rudiments of teeth have been found in 

 some Parrots. And in certain Mesozoic forms the premaxilla, 

 maxilla, and dentary bones were furnished with a complete series 

 of sharp teeth. A ring of bones is always developed in the sclerotic 

 of the eye. 



In some Mesozoic Birds the vertebral centra were amphiccelous, 

 but in all others the vertebras exhibit certain well-marked peculiari- 

 ties. Thus the neural articulations are always well developed, and 

 the arch is invariably articulated to the centrum. The neck is 

 usually very long : the number of its vertebrae ranging from eight 

 to twenty-three. The atlas vertebra forms a thin ring, in which the 

 transverse ligament may be ossified ; and the axis always has the 

 odontoid process anchylosed to it. The succeeding cervicals have 

 either short neural spines, or no spines at all ; the anterior surfaces 

 of their centra are cylindroidal and convex from above downwards, 

 and concave from side to side, the reverse condition obtaining pos- 

 teriorly (fig. 1 104). These surfaces are usually described as saddle- 

 shaped ; l and there may be a haemal spine inferiorly. In the imma- 



1 The term heterocalous has been proposed for this type of vertebral structure. 



