396 REPTILIA- ORDER II L SQ UAMA TA. 



head, body, and limbs ; but more important and constant structural peculi- 

 arities are to be found in the internal skeleton. Of the highest import is 

 the fact that in all the members of the present order the quadrate-bone is 

 movably jointed to the other elements of the skull, from which its lower 

 extremity projects. Secondly, there is no bony rod running forwards from 

 the outer side of the lower extremity of this bone to join the upper jaw ; such 

 a rod being always developed in the Crocodilia. The palate, again, has a 

 much more open structure than in either the Crocodilia or the Chelonia ; 

 not that there is any difficulty in distinguishing between a lizard or a snake 

 and a tortoise or turtle. If we turn to the backbone, equally well-marked 

 points of difference present themselves between the Squamata on the one 

 hand and the Crocodilia on the other. In the former group the ribs of the 

 chest have single upper extremities, or heads, which are articulated to 

 facets, on the sides of what are termed the bodies of the vertebrae that is 

 to say, the solid cylindrical part underlying the arch through which passes 

 the nerve-cord. On the other hand, in the Crocodilia the majority of the 

 ribs have two heads ; and those in the region of the back are articulated to a 

 long horizontal transverse projection arising from the sides of the arch of 

 each vertebrae. In the majority of the order the bodies of the vertebrae are 

 movably united by means of ball-and-socket joints : the ball being situated 

 behind and the socket in front. Neither in lizards or in serpents are the 

 teeth ever implanted in the jaws by means of distinct sockets. In all cases 

 the teeth are firmly welded to the substance of the jaw itself ; but whereas 

 in some forms they are affixed to the summits of the jaws, in others they are 

 attached to the sides of the same. To the former mode of attachment, the 

 name acrodbtd is applied, whereas the latter is termed pleurodont. Although 

 the foregoing features are amply sufficient to distinguish the Sqniamata from 

 the Crocodilia, it may be added that when the shoulder-girdle is fully de- 

 veloped, there is a breast-bone, a pair of collar-bones, and an unpaired median 

 element, with the form of the letter T, known as the interclavide. 



Sub-Order I. LacertHia. 

 LIZARDS. 



Since it has been already mentioned that the blind- worm belongs to the 

 present sub-order, it will be obvious that the presence of limbs will not suffice 

 to distinguish between the lizards and the serpents, although the majority of 

 the former are four- limbed creatures, and none of the latter have four fully 

 developed and functional legs. In the limbless snake-like lizards the tongue 

 cannot, however, be withdrawn into a basal sheath, as it is in serpents ; and 

 whereas the former are generally furnished with eyelids or external openings 

 to the ears, both these are invariably wanting in the latter. The examination 

 of the skeleton will also show that whereas in all lizards the two branches of 

 the lower jaw are joined by bone where they meet at the chin, in the serpents 

 such union is ligamentous ; and it should further be observed that the limb- 

 less lizards always retain vestiges of the shoulder- girdle. As important 

 points of distinction between the lizards and the chamaeleons, it must be 

 mentioned that such of the former as retain limbs have well-developed collar- 

 bones, and that the tongue is flattened and not dilatable. 



At the present day somewhere about seventeen hundred different species 

 of lizards are known ; these forming no less than twenty distinct families, 



