THE LACERTILIA. 217 



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 cylin- 

 droidal. A separate ossification is sometimes formed on 

 the under-sixrface of the spinal column at the junction of 

 each pair of vertebrae. Such a separate ossification, or sub- 

 vertebral wedge-bone, is commonly developed beneath and 

 between the odontoid bone and the body of the second 

 vertebra. 



The centra of the vertebrae are either procoelous, or amphi- 

 ccbIous ; the former being by far the more common condition 

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

 Splienodon, have procoelous vertebrae. The cups and balls 

 are usually ellipsoidal, tlie long axis of the ellipsoid being 

 transverse. In the Geckos, the centra of the vertebrae are 

 conicaUy 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 vertebrae of existing Lacertilia are not anky- 

 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 verte- 

 brae, and not in the intervals between adjacent vertebrae. 

 In many Lacertilia (Lacertce, Iguance, Geckos) the caudal 

 vertebrae have a very singular structure, the middle of each 

 being traversed 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 



