42 Vertebrata. 



that the Mammalia do at the present period. Only 

 four orders of reptiles are represented in the existing 

 terrestrial fauna ; at least five orders, and these in- 

 cluding the giants of the class, have perished. 



CHAPTER IX. 



LIZARDS AND SNAKES. 



24. Order 1, Lacertilia. The lizards are scale- 

 clad, and at least forelimb-bearing reptiles, with a heart 

 possessing a single ventricle, and with a lower jaw of 

 firmly united segments. The eyes are provided with 

 movable and functional eyelids, and the teeth are not 

 in sockets, but are disposed in rows either around 

 the edge or along the side of the jaws. 



The cloaca, or cavity into which the digestive 

 canal and excreting orifices open, has usually its 

 outlet placed transversely. Like most of the lowly 

 organised vertebrates, lizards display a remarkable 

 power in restoring lost parts, and in connection with 

 this we perceive in them a facility for making their 

 escape from capture by breaking off their extremities. 

 Thus a lizard taken by the tail will often break off 

 that process and escape, the fracture taking place 

 not between two of the vertebrae which make up the 

 organ, but actually through the middle of a vertebra, 

 as there is a medial cartilaginous plate in the caudal 

 vertebrae of some. In one specimen in the writer's 

 possession a lizard whose tail was cracked, but not 



