896 SUB-KINGDOM VERTEBRATA. 



looking inwards and upwards ; while those at the other extremity 

 (fig. 816, c) are termed postzygapophyses, and look downwards and 

 outwards. 1 Besides these processes for mutual articulation, there 

 are other processes on many vertebrae ; the most constant being the 

 transverse processes, or diapophyses, which may be very short as in 

 fig. 816, or greatly elongated as in fig. 817, a, d. These transverse 

 processes may arise either from the arch or from the centrum, and 

 in the trunk frequently serve for the main attachment of the ribs 

 (fig. 817, b). In fig. 816 we have an example of a vertebra with 

 very short transverse processes placed mainly on the centrum, while 

 in the vertebra of Iguanodon, figured in the sequel (fig. 1058), we 

 see very large transverse processes arising solely from the arch. 



The sides of the centrum in the anterior region of the body 

 frequently carry other articular processes for the ribs, which may 

 be termed rib or costal facets (fig. 815, cp), or parapophyses. 

 Again, the inferior or haemal surface of a vertebra may bear a 

 h<zmal spine — the term haemal being applied to the inferior as- 

 pect on account of its being directed towards the heart and large 

 blood-vessels. Each vertebral centrum carries its own arch; but 

 between the true centra of the Ichthyopsida there may be inter- 

 calated centra-like bodies, carrying no arches, and termed intercentra. 

 Rudiments of such intercentra occur in the so-called wedge-bones or 

 h<z??iapophyses, found between the lower borders of the centra in 

 certain Reptiles, and more especially between the first and second 

 vertebrae. Further, the Y-shaped chevron-bones, usually articulating 

 between adjacent vertebral centra in the tail of many Vertebrates, 

 are also morphologically intercentral elements. In Fishes the ver- 

 tebral column can only be divided into trunk and caudal regions ; 

 but in the majority of higher forms further divisions can be made. 

 Thus the vertebrae of the neck, in which the ribs never articulate 

 with the sternum, are termed cervical (fig. 818, c) ; the first of this 

 series being designated the atlas, and the second the axis. It is 

 further remarkable that in most instances the centrum of the atlas 

 is not joined to its arch, but either remains as a distinct element, or 

 becomes anchylosed to the front of the centrum of the axis to form 

 the so-called odontoid process ; and it appears that in the latter case 

 the inferior bar connecting the two lateral arches of the atlas is 

 really the remnant of the first intercentrum. It is, moreover, prob- 

 able that this separation of the component elements of the atlas 

 should be regarded as a retention of the primitive feature obtaining 

 in the vertebral column of many extinct Fishes and Amphibians, 

 where, as will be noticed below, the whole of the vertebrae are thus 

 disintegrated. Finally, it has been suggested that certain bony 



1 These are well seen in the vertebra of Calamospondyhts above mentioned. 



