134 ZOOTOMY. 



little from the anterior or true thoracic, save for the fact that 

 their centra are somewhat shorter. 



1 8. The sacral vertebrae, distinguished by their short 

 centra, and by the presence of large, expanded, outwardly 

 directed transverse processes, against which the ilia 

 ( 10 1 ) abut : these processes are formed as distinct 

 ossifications, and represent sacral ribs. The curvature of 

 the posterior face of the centrum of the first, and of the 

 anterior face of the centrum of the second sacral vertebrae, 

 is somewhat less marked than in the other vertebrae : the 

 posterior convexity of the second sacral is more than usually 

 pronounced. 



19. The anterior caudal vertebrae resemble the sacral, 

 but have longer centra, slenderer transverse processes, and 

 longer neural spines. 



20. The chevron bones, attached to the ventral faces 

 of the centra of many of the anterior caudal vertebrae : each 

 is Y-shaped, the paired upper limbs of the Y articulating 

 with facets immediately beneath the posterior convexity of 

 the centrum, while the stem or lower-limb is free and takes 

 a direction downwards and backwards. 



21. The posterior caudal vertebrae undergo a gradual 

 simplification of structure towards the distal end of the tail, 

 being finally reduced to small rod-like centra. 



In all the caudal vertebrae but a few of the anterior ones, there is 

 a transverse vertical zone of the centrum which remains unossified, and 

 along which the vertebra readily separates : many lizards lose their 

 tails by the parting of the tissues across one of these weak places, and 

 a new tail is formed, the supporting axis of which is not divided into 

 vertebrae, but consists of an unjointed calcined rod. 



IV. Make out the characters of the ribs and sternum 

 as follows : 



22. The sternum, (Fig. 39, p. 157, st), a rhomboidal plate 



