THE LIZARDS 2465 



process arising from the arches of the vertebrae; from the tortoises, where the single- 

 headed ribs articulate at the junction between the bodies of the two vertebrae; 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 vertebrae, as 

 shown in the figure on p. 2363, 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 the 



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

 acteristics 



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 jaw- 

 bones, when the detention 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 longi- 

 tudinal, 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 breastbone 

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

 The above being the leading characteristics of the entire order of 

 Special scaled reptiles, it remains to consider how the lizards (L/acertilia) are 



, to be distinguished from the other two suborders into which the ex- 

 tcnstics ot 

 Lizards isting 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 dis- 

 tinguishing between a lizard and a snake. The matter is, however, somewhat com- 

 plicated by the circumstance that certain lizards, like the familiar slowworm, 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, al- 

 though 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 chamaeleons 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 



