VERTEBRAL COLUMN 47 



In Snakes and some Lizards (Hatteria, Iguana) extra articular 

 processess (zygosphenes and zygantra) are developed on the neural 

 arches. In Hatteria and the Geckos, small separate ossifications 

 (intercentra, comp. p. 43, 44, 46) are present on the ventral side- 

 of the vertebra] column between many of the centra. In the caudal 

 region of Lizards anunossified septum remains in the middle of each 

 centrum, so that the tail easily breaks off at these points : when this 

 happens the tail grows again, but proper vertebrae are not formed. 



In fossil Reptiles, which both as regards size and number of species 

 usually surpassed the existing representatives of the class, the sacrum often 

 consisted of as many as four or five vertebrae. The following facts will give 

 some idea of the monstrous proportions of these old genera of Reptiles : 

 Atlantosaunis immanis, a North American Dinosaur, reached a length of 



Po 



FIG. 35. ANTERIOR PORTION OF THE VERTEBRAL COLUMN OF A YOUNG 



CROCODILE. 



WK, centrum ; Ob, neural arch ; Ps, neural spine ; J-sr, intervertebral disc ; Pt r 

 transverse process, arising from the base of 'the arch and articulating with 

 the rib (jff, R l , R*) at f ; A, atlas ; u, ventral element, and 8 arch of atlas ; o, 

 "pro-atlas"; Ep, axis, articulating with the atlas at h; Po, odontoid 

 process. 



about 80 feet, and the transverse diameter of the individual vertebra* 

 amounted to 16 inches, while Apatosaurus laticollis, found in the same strata, 

 possessed cervical vertebrae which reached a diameter of 3^ feet. 



A knowledge of fossil genera of Reptiles is of the greatest interest, as in 

 many groups important points of connection with Birds can be recognised. 



Birds. The vertebral column of Birds corresponds with that 

 of Reptiles not only in its phylogenetic relations, but also onto- 

 genetically. In both groups the notochord eventually disappears 

 entirely, and the whole skeleton becomes strongly ossified. 

 Archaeopteryx, as well as Ichthyornis (from the American Cre- 

 taceous), possessed biconcave vertebrae, but in existing Birds this 

 character never occurs except in the free caudal vertebrae (p. 49). 

 Cervical, thoracic, lumbar, sacral, and caudal regions can be distin- 

 guished. The arches always become united into a single mass 

 Avith the corresponding centra, and are no longer separated from 



