THE LACERTILIA. 187 



of vertebrse ; and, except in the AmphisbmncB and some few 

 other Lizards, the tail is long. Those Lizards which possess 

 hind-limbs have a sacrum, into which not more than three 

 vertebrae, and rarely more than two, enter. The presacral 

 vertebrae are distinguishable, when sternal ribs are present, into 

 cervical and dorsal. All those vertebrae which lie in front of 

 the first sternal rib are cervical ; and if, as sometimes happens, 

 the last two or three dorsal vertebrae are devoid of ribs, they 

 become lumbar. Not more than nine vertebrae are met with 

 in the cervical region of existing Lacertilia, and this number 

 is rare. The number was greater in some extinct Lacertilia. 



The atlas is composed of three pieces, one inferior and 

 two superolateral. The odontoid bone is closely united with 

 the second vertebra, and its anterior face may be cylindroidal. 

 A separate ossification is sometimes formed on the under-sur- 

 face of the spinal column at the junction of each pair of ver- 

 tebrae. Such a separate ossification, or sub-vertebral wedge- 

 bone, is commonly developed beneath and between the odon- 

 toid bone and the body of the second vertebra. 



The centra of the vertebra are either proooelous, or amphi- 

 coelous ; the former being by far the more common condition 

 in existing Lacertilia, all of which, except the Geckos and 

 Sphenodon, have procoelous vertebrae. The cups and balls 

 are usually ellipsoidal, the long axis of the ellipsoid being 

 transverse. In the G-eokos, the centra of the vertebrae are 

 conically excavated at each end ; and, except in the centre of 

 each vertebra, where it is ossified, the notochord persists 

 throughout the spinal column. 



The sacral vertebrse of existing Lacertilia are not anchy- 

 losed together, nor are their articular faces modified, the two 

 being connected by a free cup - and - ball articulation. The 

 movements of the two vertebrae, however, are restrained by 

 the strong ligaments which connect their neural spines and 

 arches, and by the fibro-cartilage which connects and covers 

 the free ends of their expanded ribs. 



In the anterior part of the tail of the Lacertilia there are 

 usually well-developed subvertebral chevron bones, which are 

 commonly attached to the bodies of the several vertebrae, and 

 not in the intervals between adjacent vertebrae. In many 

 Lacertilia [Lacertm, Iguanm, Geckos) the caudal vertebrae 

 have a very singular structure, the middle of each being trav- 

 ersed by a thin, unossified, transverse septum. The vertebra 

 naturally breaks with great readiness through the plane of the 

 septum, and when such Lizards are seized by the tail, that 



