306 COMPARATIVE ZOOLOGY OF CHORDATES 



little elements called intercentra which represent the hypo- 

 centra. These intercentra and haemal arches in the amniotes 

 are called " chevron-bones " ; they are never found in the 

 amphibia. Primitive reptiles like Seymouria and Sphenodon 

 have a complete set of intercentra all the way along the vertebral 

 column, and some primitive mammals (hedgehog ; mole) 

 have intercentra in the lumbar region. 



The transverse processes are lateral extensions of the 



Fig. 155. — View of the anterior region of the vertebral column of the croco- 

 dile seen from the left side. 



C2, centrum of the 2nd or axis vertebra ; hi, hypocentrum of the 1st or 

 atlas vertebra ; mi, neural arch of the 1st or atlas vertebra ; M2, neural arch 

 of the 2nd or axis vertebra ; op, odontoid peg, or pleurocentrum of the atlas 

 vertebra which has become attached to the axis vertebra ; pa, proatlas ; 

 n, 2, 3, ribs of the 1st to 3rd vertebrae ; V3, 4, 3rd and 4th vertebrae. 



vertebrae from the base of the neural arches. The dorsal (or 

 tubercular) head of the ribs is attached to the transverse process, 

 and in all land-vertebrates except the most primitive Stego- 

 cephalian amphibia (such as Eogyrinus),the transverse processes 

 of at least one vertebra are attached to the ilia of the pelvic 

 girdle forming the sacrum. The sacrum is of course not formed 

 in animals which do not possess hind limbs. 



The first vertebra in amphibia is modified to carry the head, 

 and the vertebral column (which in fish is divisible only into 



