586 REPTILIA [CH. 



some species have lost their limbs. The kind of life to which a 

 snake-like form is suited is a lurking one amongst crevices in 

 stones, or thick vegetation, or in the soil, where movement is best 

 effected by wriggling and limbs would be in the way. 



Under these circumstances, we must either class together all 

 limbless Sauria as Snakes, and thus give up the idea that the 

 members of an order must necessarily be descended from the same 

 ancestral species, or else we must select one group as the true 

 Snakes (Ophidia), the members of which have some other characters 

 in common besides the negative one of having no limbs. This 

 latter course is that which has been adopted by Huxley, who defines 

 true Snakes somewhat arbitrarily as those forms in which the two 

 halves of the mandible are connected by elastic fibres, so that they 

 can be widely divaricated from one another in the act of swallowing, 

 and which have lost all trace of the pectoral girdle and of the 

 urinary bladder, although they may retain traces of hind-limbs. 



Sub-order 1. Lacertilia. 



The Lacertilia then include all species of Sauria which have the 

 right and left halves of the mandibles connected by a sutural 

 symphysis and which retain a urinary bladder and some trace of the 

 pectoral girdle. In all other characters they are a very diversified 

 group. Most of them possess well developed limbs, movable 

 eyelids and movable quadrate bones, but a good many species 

 belonging to specialised burrowing families have no limbs and 

 scarcely a trace of the pectoral girdle, while the eyes are concealed 

 beneath the skin and the quadrate has become more or less im- 

 movable. Some, e.g., Draco volans, have the hinder ribs expanded 

 so as to press out two expansions of skin and form a parachute-like 

 expansion on each side, by means of which they are supported as 

 they flit from tree to tree in great leaps. Most feed on insects, 

 worms, etc. like the English Lizards ; some are large enough to seize 

 mice and birds and frogs. The limbless forms are represented in 

 England by the Blind- or Slow-worm, Anguis fragilis, and in North 

 America by the allied Glass-snake, Ophisaurus ventralis. These 

 animals have skulls like that of Lacerta and rudiments of pectoral 

 girdles. Besides the Blind-worm, the Common Lizard, Lacerta 

 vivipara, and the Sand-lizard, Lacerta agilis, are British. 



In North America four families of Lizards are represented, one 

 being that of the limbless AUJGUIDAE, while the most remarkable 



