116 STRUCTURE AND CLASSIFICATION OF BIRDS 



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

 are, as has been shown, 1 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 

 Aptery.T 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 



"\ --sss^^Tv ^ ie ma ^ter. They are especially 



conspicuous among the Limi- 

 colae 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 

 j^'^^S&&& 1 * vertebrae 1-5. Behind these 



FIG. 65. LAST Two VERTEBRA OF are a series of hypapophyses, 



STKUTHIO (ATTEK MIVAI.T). which ^ & contmuatlon of 



us, neural spines ; rf, osseous bridge. 



the same series, but much more 

 pronounced and ankylosed to the vertebrae. 



They exist also in the duck tribe. In Biziura lobata 

 there are three distinct intercentra in the form of largish 

 nodules. I have found intercentra also in Palamedeae, Tubi- 

 nares, Steganopodes, Colymbi, Herodiones, Opistlwcomus. 



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 



1 This matter of the composition of the vertebra has been recently gone into 

 by G.VDOW (on the 'Evolution of the Vertebral Column of Amphibia and Amniota,' 

 Phil. Trans. IH'.Mi, p. 1), who quotes previous literature. 



