112 STKUOTUEE AND CLASSIFICATION OF BIRDS 



entirely exceptional among birds. Steatornis is another bird 

 with opisthocoelous vertebrae. So too are the cormorants and 

 darters. Another form of vertebral articulation met with 

 in reptiles — namely, the proccelous articulation — also exists 

 in birds. In all birds, of course, the atlas is proccelous, 

 articulating with the convex occipital condyle. In many 

 birds ' the last two movable joints in the caudal series 

 become proccelous.' 



The biconcave form of vertebrae characterises the extinct 

 Archceopteryx,^ and the gull-like Ichthyornis, called by its 

 name on this very account. The concavities, however, 

 according to Fuebeingbr, seem rather to have been produced 

 by the maceration out of a plug of cartilage than to have 

 characterised the unaltered vertebra. It is doubtful, in fact, 

 whether the spaces left in the fossil vertebrae were filled 

 during life with copious remains of the notochord, as in 

 fishes. 



The vertebral column of birds can be distinguished into 

 four series, as in the higher vertebrates generally. It is 

 customary to regard as cervical those vertebrae which'either 

 have no movable ribs or, if they have, do not become con- 

 nected through their intermediary with the sternum. The 

 rib-bearing vertebrae are the dorsal series, while those which 

 articulate with the pelvis are usually termed sacral.. But 

 it seems better to reserve the term ' sacral ' for the two 

 vertebrae which, in the chick, bear the ilium. 



The number of true sacrals is not, however, always two. 

 J. J. Paekee ^ describes in the young Apteryx three vertebrae, 

 which abut upon the ilium, and are the only ones in which 

 there are separate rib-like ossifications at the ends of the 

 transverse processes. These vertebrae, which are quite con- 

 spicuous in the adult (fig. 62), are regarded as the true 

 sacrals. There are also three in the ostrich (fig. 63). In 

 other birds (e.g. Larus, Ghionis) there is apparently only 

 one sacral vertebra. , 



Behind the sacrum are the caudal vertebrae. Archao- 

 pteryx is unique among birds for its long tail, composed of 

 ' Not certainly. ■' ' Development of Apteryx,' Phil. Trans. 1891. 



