42 VERTEBRATE SKELETON 



fossil reptiles. In other Sauropsida where the development is known, inter- 

 vertebral cartilage like that in Amphibia, occurs, interrupting the notochord in 

 the same way, and then these are cut, partially or completely, and in such a 

 manner that either pro- or opisthocoelous centra result, the notochord persisting 

 for a time in the centra. In mammals, where development is imperfectly 

 known, the chorda disappears, first, in the centra, but may persist through life 

 in the fibro-cartilage discs (menisci) which separate the successive centra. 



Haemal ribs are not known in Amniotes, except as their homo- 

 logues, the chevron bones persist in the tail. Pleural ribs, either 

 bicipital or with a single head, are always present, attached to the 

 thoracic vertebrae and to more or fewer of the cervicals, while in 

 the sacral region sacral ribs connect the vertebrae with the ilium of 

 the pelvis. When a sternum is present, the anterior thoracic ribs are 

 connected directly with it, while those farther back are connected 

 with those in front or end freely. 



REPTILIA. — ^All types of vertebrae — amphi-, pro-, opisthoccelous 

 and amphiplatyan — occur in reptiles, but usually one kind prevails 



Fig. 49. — Anterior and posterior faces of vertebra of Python, c, centrum; poz, postzy- 

 gapophysis; prz, prezygapophysis; za, zygantrum; zs, zygosphene. 



in any one individual. Cervical, dorsal, sacral and caudal regions 

 are well marked, except in limbless forms (snakes, apodal lizards 

 Pythonomorphs), there being usually two sacrals in most living spe- 

 cies (occasionally three in some lizards and Crocodihans, and rising 

 to six in some fossils). Nearly all presacral vertebrae bear ribs, 

 although they are often lacking on atlas and epistropheus, and in 

 Lacertilia and Crocodilia lumbar vertebrae are differentiated from 

 the other dorsals (thoracics) by the absence of ribs., Usually the 



