ROOM III. VERTEBRAE OF SAURIANS. 165 



for the passage of the large blood-vessels which supply the 

 tail. 



In most of the existing reptiles, (as for example the Croco- 

 dile, Iguana, &c.) the bodies of the vertebrae are concave in 

 front and convex behind, 1 forming a series of ball and socket 

 joints; but in many extinct genera both the articulating 

 surfaces are flat, or slightly concave ; 2 or flat in front and 

 concave behind. 3 



In quadrupeds the annular part is anchylosed to the ver- 

 tebral centre ; in reptiles it is generally united by suture ; 

 but all traces of this mode of connexion are often obliterated 

 in aged individuals. By reference to Lign. 35, and its de- 

 scription, the form, arrangement, and connexion of the dif- 

 ferent vertebral elements in certain fossil reptiles, may be 

 easily comprehended. The bones in the spinal column of 

 the same animal are considerably modified in the several 

 regions of the neck (cervical), back (dorsal), and tail (caudal). 

 The cervical are generally of the most complicated structure, 

 and the caudal, the most simple. 



From this exposition, the reader will perceive that every 

 vertebra consists of the following essential parts, namely, the 

 body, or centrum, and the annular part, or neurapophysis, so 

 termed, because it protects the nervous chord ; while a caudal 

 vertebra has, in addition, the chevron-bone, called also hcema- 

 pophysis, from its affording a passage to the large blood- 

 vessels. 



The bodies of the vertebrae are in general solid, and con- 

 sist of the ordinary osseous structure ; but in certain fossil 

 reptiles the centre of the bone is filled with calcareous spar, 

 indicating an irregular medullary cavity ; but this structure 

 also obtains in the caudal vertebrae of mammalia, for example, 

 in our domestic Ox. 4 



1 Named by Professor Owen proccdian vertebrae; from two Greek 

 words, signifying concave before. 



2 Amphicaelian, concave at both ends. 



3 Platycodian, flat in front, concave behind. 



4 I am not aware that this fact was noticed by any anatomist, till 

 pointed out by me in a Lecture on the Structure of Fossil Saurians, 

 delivered in the LONDON INSTITUTION. As some Palaeontologists have 

 ascribed a specific and even generic value to this character, I annex 

 figures of transverse and longitudinal sections of a vertebra of the 



