294 



ZOOLOGY 



SECT. 



surface of the trunk they are small, hexagonal, and indistinctly 

 keeled. On the ventral surface they are larger and are arranged 

 in eight longitudinal rows. Immediately in front of the cloacal 

 aperture is a large pre-anal plate. A collar-like ridge of larger 

 scales surrounds the throat. On the tail the scales are elongated, 

 keeled, and arranged in regular transverse (annular) rows, giving 

 the tail a ringed appearance. On the surface of the limbs the 

 scales of the pre-axial side are larger than those of the post-axial. 

 The scales on the upper surface of the head (head shields) are 

 large, and have a regular and characteristic arrangement. 



Endoskeleton. The vertebral column is of great length and 

 made up of a large number of vertebrae. It is distinctly marked 

 out into regions, a cervical of eight vertebrae, a thoracico-lumbar of 

 twenty-two, a sacral of two, and a caudal of a considerable, but 



indefinite, number. A 

 vertebra from the an- 

 terior thoracic region 

 (Fig. 906, A, B) pre- 

 sents the following lead- 

 ing features. The cent- 

 rum (cent.) is elongated 

 and strongly procoelous, 

 i.e. the anterior surface 

 is concave, the posterior 

 convex; the neural arch 

 bears a short neural 

 spine (sp). There are 

 pre- and post-zygapo- 

 physes (pry t pty) t ihe 

 former with their arti- 

 cular surfaces directed 

 upwards, the latter downwards. On each side at the junction of 

 centrum and neural arch is a facet the capitular facet for the 

 articulation of a rib. The cervical vertebrae in general are similar 

 in essential respects to those of the trunk, but are somewhat shorter. 

 The first two, however, differ greatly from the others. The first 

 is the atlas ((7, D). It has no distinct centrum, but is in the form 

 of a ring ; ventrally on its anterior face it bears a smooth articular 

 facet for the occipital condyle of the skull. It consists of three 

 distinct ossifications, one ventral, the others dorso-lateral : the 

 latter do not quite meet dorsally, being separated by a space 

 bridged over by membrane. The second or axis (E) has a short 

 conical process the odontoid process (oe) projecting forwards 

 from its centrum. In the natural position of the parts the 

 odontoid process, which is a part of the centrum of the atlas, and 

 is not actually fused with, though firmly fixed to, the axis, lies 

 in the lower or ventral part of the opening of the atlas, separated 



FIG. 906. Vertebrae of Lizard. A, anterior, B, posterior, 

 view of a thoracic vertebra ; C, lateral, 2), anterior, view 

 of atlas vertebra ; E, lateral view of axis. cent, centrum ; 

 hyp. hypapophysis of axis ; lat. lateral piece of atlas ; 

 lig. ligamentous band dividing the ring of the atlas 

 into two ; neur. neural arch of atlas ; od. odontoid pro- 

 cess ; pr. zy. pre-zygapophysis ;pt. zy. post-zygapophysis ; 

 rb. rib ; sp. spine ; rent, ventral piece of atlas. 



