LIZARDS. 107 



long process arising from the arches of the vertebrae ; from the tortoises, where the 



single-headed ribs articulate at the junction between the bodies of two vertebras ; 



and from the plesiosaurs, in which the single-headed ribs of the back are articulated 



to processes or facets on the arches of the vertebras. In most of the members of 



the order the body of each vertebra has a cup in front and a ball behind, by which 



it articulates with the adjacent segments of the column — an arrangement paralleled 



among modern crocodiles. In some lizards, and in all snakes, the vertebras, as 



shown in the figure on p. 6, have additional surfaces on their arches for mutual 



articulation, thus communicating additional flexibility, and at the same time strength 



to the backbone. 



Another important feature in which the order differs from all 



Other Characters. , „ _ i-i-t 



the preceding ones, is the absence or any system of true abdominal 



ribs, or of their equivalent, a plastron, on the inferior surface of the body. As 



regards the teeth, these differ from those of the orders hitherto considered in that, 



instead of being implanted in separate sockets, they are firmly soldered to the bones 



of the jaw. . In some cases they are attached to the very summit of the jawbones, when 



the dentition is said to be acrodont ; while in others they are affixed to one of the 



side- walls of the free edges of the jaws, the term pleurodont being then employed. 



Another divergence from both crocodiles and tortoises is to be found in the vent 



opening by a transverse aperture, whereas in the former group it is longitudinal, 



and in the latter either circular or longitudinal. Finally, in those forms in which 



the bones of the chest attain their fullest development, there is a breast-bone or 



sternum, a pair of collar-bones or clavicles, and a median T-shaped interclavicle. 



Special The above being the leading characters of the entire order of 



Characters of scaled reptiles, it remains to consider how the lizards (Lacertilia) are 



Lizards. ^ Q -j^ distinguished from the other two suborders into which the 

 existing members of the assemblage are divided. Externally, by far the greater 

 number of lizards are four-limbed reptiles of a crocodile-like appearance, with the 

 head, neck, body, and tail well distinguished from one another, and if we had these 

 alone to deal with, there would be no sort of difficulty in distinguishing between a 

 lizard and a snake. The matter is, however, somewhat complicated by the circum- 

 stance that certain lizards, like the familiar slow-worm, lose all external traces of 

 limbs, and assume an elongated snake-like form, with the head passing imperceptibly 

 into the body without the intervention of a distinct neck, and without any external 

 indication of where the body ends and the tail commences. Externally, such snake- 

 like lizards are very difficult to distinguish from snakes, but on opening the mouths 

 of the former it will be found that the tongue cannot be withdrawn into a sheath 

 at its base, as is always the case with the latter. Further help in discriminating 

 between the two is afforded by the circumstances that whereas snakes have neither 

 eyelids nor external ear-openings, both these are usually, although not invariably, 

 present in the limbless lizards. As additional distinctive features of the present 

 group, by means of which they can be distinguished both from snakes on the one 

 hand and from chamseleons on the other, the following points may be noticed. In 

 all lizards the two branches of the lower jaw are united at the chin by means of a 

 bony suture ; while in all the species furnished with limbs collar-bones are present ; 

 and when the limbs are absent, some traces of the bones forming what is known 



