116 STRUOTUBE AND OLASSIFIOATION OF BIRDS 



discs,' the ' intercentra ' or ' basiventral ' elements. These 

 are, as has been shown,' originally the portions of each 

 vertebra with which the ribs articulate, from which they 

 are outgrowths. But as the ribs come to articulate with the 

 centra these structures degenerate. In the development of 

 Apteryx T. J. Parker found a postoccipital and a post- 

 atlantal intercentrum, and two in the caudal region, which 

 ossify so as to retain their independence in the adult 

 skeleton. 



Intercentra in the caudal region of the bird's vertebral 

 column are by no means so rare as might be inferred from 



some published statements upon 

 the matter. They are especially 

 conspicuous among the Limi- 

 colse and the nearly allied auks, 

 and in most water birds. In 

 Numenius femoralis, -for exam- 

 ple, there are three small osseous 

 nodules lying between caudal 

 vertebrae 1-5. Behind these 

 Fig. 65.— Last Two Veetebb*; of are a series of hypapophyses, 



StBCTHIO (aFTEE MiVAKT). „i • n i" i- J! 



, . ^ , ^ . wnich are a contmuation of 



ns, neural spines ; a, osseous bridge. 



the same series, but much more 

 pronounced and ankylosed to the vertebra. 



They exist also in the duck tribe. In Biziura lohata 

 there are three distinct intercentra in the form of largish 

 nodules. I have foimd intercentra also in Palamedese, Tubi- 

 nares, Steganopodes, Colymbi, Herodiones, Opisthocomus. 



These free intercentra are rare among the Pico-passeres, 

 but in a few of them are present. Thus in Tcccus there, is 

 a distinct intercentrum lying between the last free caudal 

 vertebrae. 



The hawk tribe have not these bonelets as distinct struc- 

 tures. 



In the cuckoos, parrots, Ralli, Otides, Columbae, and the 



' This matter of the oompositidu of the vertebra has been recently gone into 

 by Gadow (on the 'Evolution of the Vertebral Column of Amphibia andAmniota,' 

 Phil. Trans. 1896, p. 1), who quotes previous literature. 



