Class 5. Aves. 437 



tbe number of free caudal vertebrae* is smallj usually from six 

 to eight ; the terminal bone^ howeverj which is much compressed, is 

 a fusion of several short vertebrae, separate in young Birds. The tail 

 vertebrae are, like the tail itself, short; though in Archaeopteryx 

 (Fig. 378) there was a long, thin tail of a reptilian type, and con- 

 sisting of a number of vertebrae, some of which, were elongate. 

 Short ribs are attached to tjhe cervical vertebrse, and like 

 the thoracic ribs have two articular processes; they are fused 

 in the adult, though separate in young forms. The cervical ribs 

 gradually increase in length posteriorly, and remain separate 

 throughout life, thus affording a transition to the thoracic ribs,t 

 which consist of two bony portions connected at an angle; the ventral 

 portion is attached to the sternum, and from the hinder edge of the 

 dorsal part arises a narrow oblique process {processus micinatus), 

 which, overlaps the next rib; in young Birds it is a separate bone. 

 The sternum is completely ossified and very large, covering the 

 greater part (or at least a large part) of the ventral region of the 

 body. It is almost always furnished with a large projecting keel, to 

 which some of the muscles of flight are attached, and which is only 

 absent from certain forms with rudimentary wings {e.g., Eatitae) ; 

 here the sternum itself is smaller than usual. Posteriorly it is often 

 perforated or notched on either side, the gaps being covered with 

 connective tissue. There is no episternum. 



The skull usually resembles that of the Reptiles very closely ; 

 among living forms, especially that of the Lizards ; there is only 

 one occipital condyle; the quadrate is reptilian, as also the 

 conditions of the palatine and pterygoid; the region between 

 the large orbits is compressed into a perpendicular bony lamina, 

 the interorbital or orbital plate, which maybe partially 

 membranous. The prominent premaxillee, which fuse early to 

 form a single bone, are characteristic of Birds ; they form the entire 

 edge of the beak, and also send a long branch, almost to the frontals, 

 between the external nares. The maxilla, on the other hand, are 

 relatively small, and lie within the posterior portions of the pre- 

 maxillffi. The lower end of the large, very movable quadrate 

 is connected with the beak (the premaxilla and maxilla) by a bony 

 bridge formed of the pterygoid posteriorly, the palatine anteriorly ; 

 the pterygoid and palatine are both elongate bones, and in most 

 liirds they slide upon the thickened lower rim of the orbital plate 

 mentioned above at their point of contact. From the lower end of 

 the quadrate to the beak there runs yet another bony bridge, the 



* The caudal centra are Wplanar. 



t Usually, the first vertebra bearing ribs attached to the sternum, is termed the 

 first thoracic ; the transition from cervicals to thoraoics is, however, quite gradual, 

 and this distinction is artificial. 



