44 VERTEBRATE SKELETON 



There are rarely more than nine cervical vertebrae (a few cretaceous lizards 

 have more); and in serpents, where the first two vertebrae lack ribs, there is no 

 other distinction between cervicals and dorsals. In Ophidia the ribs begin on 

 the third vertebra and continue to the tail. Atlas and epistropheus articulate 

 by a dens epistrophei fused with the epistropheal centrum in Ophidia, but 

 separated from it by a suture in Lacertilia. In many Squamates there is no 

 distinction of thoracic and lumbar regions. The ribs of snakes and geckos are 

 articulated to a single transverse process which, in geckos, has a double condyle 

 for the rib. The ventral ends of the ribs are free in snakes and, in conjunction 

 with the ventral scutes, are important in locomotion. Several of the anterior 

 ribs of lizards are connected with the sternum (fig. 6i) and in geckos and Chame- 

 leons the posterior ribs of the two sides fuse in the middle line. The dorsal part 

 of each rib is ossified in lizards, the ventral remaining cartilage, a dift"erentiation 

 of vertebral and sternal ribs. Draco has the last five or six ribs long and straight, 

 supporting a fold of skin which serves as a parachute. Laccrta has thoracic and 

 lumbar vertebrae differentiated. 



In correlation with the reduction of the hind Hmbs in Ophidia, Pythono- 

 morphs and apodal lizards, no sacrum is differentiated in these groups. Other 

 lizards have two separate sacrals. The caudal vertebrae bear chevron bones, 

 usually articulated to the centra. 



Chelonia. — In turtles the vertebral column is relatively short and no lumbar 

 region occurs. The very flexible neck contains eight vertebrae, one or two of 

 which have biconvex centra; the others are pro-, opistho- and amphicoelous. 

 The atlas consists of neurapophysis and subchordal bar, its centrum being 

 joined by that suture of the epistropheus to form the dens. In other vertebrae 

 neural arch and centrum are joined suturally, at least in the young. There are 

 distinct cervical ribs in the embryo, which fuse almost completely with the 

 vertebrae and in some genera are so reduced that they are said to be absent. 

 Of the ten thoracic vertebrje the first is free, the others immovably united to 

 each other, and, except in the Atheca, to the neuralia of the carapace. (Gotte 

 claims that the neuralia are not dermal but are developed from the periosteum 

 of the neural spines.) 



The first dorsal vertebra bears a short rib which joins the under side of the 

 carapace near the second rib. The next eight ribs are greatly expanded and 

 extend laterally to the margin of the carapace. They are articulated to the 

 anterior ends of the centra in the posterior part of the series, farther forwards 

 their heads are intervertebral. The tenth dorsal vertebra has a very short rib, 

 suturally united to both centrum and carapace. There are two amphicoelous 

 sacral vertebrae, each with its short pair of sacral ribs, connected by suture with 

 both centra and carapace, the sacrum being increased by the inclusion of pre- 

 and postsacral vertebrae (synsacrum). The sixteen to thirty caudal vertebrae 

 are usually procoelous, sometimes opisthoccelous. Their transverse processes 

 join the vertebrae between neural arch and centrum and chevrons are greatly 

 reduced or absent. 



Crocodilia have all five vertebral regions well marked. The oldest mem- 

 bers of the order had amphicoele vertebrae, later came those with amphyplatyan 



