258 



AVES 



C'LA^;S IV 



ostrich, which as well as the apteryx has three sacrals. The gieater iiumber 



of vertebrae in the synsacrum are united with the ilia by diapophyses, or 



transverse processes given otf from the neural arch. The anchylosed vertebrae 



behind the true sacrals are the urosacrals, and belong 



to the caudal series ; their number may vary slightly 



within specific limits. The free caudals in existing Jma 



birds usually number about six, besides the terminal 



ploughshare-bone or pygostyle, which is composed of 



from four to six fused vertebrae. In the Jurassic 



äj Pt 



Fig. 3(54. 



Otts tarda, Linn. Recent ; Europe. Lateral (A), aud palatal (ß), a.spects 

 of skull, i/j. als, Alisphenoid ; ang, Angular ; art, Articular ; bo, Basi- 

 occipital ; bt, Basitemporal ; c, Occipital condyle, underneath foraiuen 

 magnuin ; d, Dentary ; et. Median ethmoid ; fr, Frontal ; j, Jugal ; jmx, Pre- 

 niaxilla ; l, Lachrymal ; mx, Maxilla ; N, External nares ; ol, Exoccipital ; 

 OS, Supraoccipital ; pa, Parietal ; pal, Palatino ; pt, Pterygoid ; q, Quadrate ; 

 sm. Interorbital septum ; spb, Basisphenoid ; sq, Squaniosal ; vo, Vonier (after 

 Claus). 



Archaeopteryx there were at least twenty free and long caudals with a pair of 

 rectrices to each vertebra. 



All the presacral vertebrae with the exception of the atlas are usually 

 costiferous, and ribs may be present also on the anterior two or three sacrals ; 

 those of the cervical region fuse with the vertebrae, except the last two or 

 three. The true character of the cervical processes thus formed may be seen 

 in embryos and to particular advantage in the young ostrich, where they 

 remain for some time free, as they did permanently in Archaeopterijx. With 

 few exceptions the last cervical rib and all the thoracic ribs save the last one 

 or two bear uncinate processes, or thin fiat blades of bone directed obliquely 

 upward and backward to overlap the succeeding rib. These processes usually 

 unite with the ribs, but in some cases (Moas and many water birds) remain 

 free. The screamers, Chauna, Palarfiedea^ are exceptional in that the ribs are 

 devoid of uncinate processes, while they are vestigial in the Secretary Bird. 

 Appended to the thoracic ribs are sternal or abdominal ribs, from two to 

 five pairs of which are attached to the sternum. 



A sternum is always present, except possibly in Archaeopteryx, and this is 

 externally convex and generally broad. It may cover only the anterior 

 portion of the thorax, or may be so prolonged, as in water birds, that its 

 posterior end underlies the pelvis, or extends nearly to the pubes, as in 

 humming-birds. In birds of fiight or those which use their wings in 

 swimming, as the penguins and auks, the sternum is keeled beneath ; but 



