56 The Alligator and Its Allies 



two articular surfaces, an anterior and more medial 

 one for articulation with the head of the rib (3) and 

 a posterior and more distal one for articulation with 

 the tubercle of the rib. These two surfaces ap- 

 proach each other as the vertebras are followed 

 caudad until, in the last two thoracic vertebrae, 

 they form practically one surface. The processes 

 of the pre- and postzygapophyses spring from the 

 arch at the base of the transverse process; the 

 former surface is directed dorsally and medially, 

 the latter ventrally and laterally. The interverte- 

 bral foramina are smaller and more nearly circular 

 than in the cervical region, and are more closely 

 surrounded by bone. 



The Lumbar Vertebrce. The five lumbar vertebras 

 are essentially like the thoracic except that the 

 transverse processes, which, of course, bear no ribs, 

 are both shorter and narrower. The postero- 

 lateral border of the centrum of the last of these 

 five vertebrae has a small surface for articulation 

 with the antero-medial border of the transverse 

 process of the first sacral vertebra. 



The Sacral Vertebrce (Fig. 18, B). These are 

 two in number. The centrum of the first is con- 

 cave in front and flat behind, instead of being 

 convex behind, and the second is flat (instead of 

 concave) in front, and convex behind. The neural 

 spine and zygapophyses are as in the lumbar region. 

 Projecting laterally from each sacral vertebra, 

 forming a close, sutural joint with both centrum 



