THE ORDER LACERTILIA 9 



present purpose if we state that the Sauria are the 

 most recently developed reptiles, and include both 

 the Ophidia or Serpents and the Lacertilia or Lizards. 

 Looking now at the definition above it becomes 

 apparent that lizards are distinguished from serpents 

 by the fact that in the former the two halves of the 

 lower jaw are firmly united together by a bony 

 suture or joint which allows of no distension. In 

 the serpents, on the other hand, these two halves of 

 the lower jaw are connected by an elastic ligament 

 which allows of immense distension, an anatomical 

 difference which we shall find has its functional 

 counterpart in the different kinds of food swallowed. 

 It is only necessary, further, to have a clear idea of 

 what constitutes a Eeptile. Briefly, Eeptiles may be 

 described as the only vertebrate animals which are 

 cold-blooded, which breathe by lungs throughout life, 

 and which have the skull joined on to the vertebral 

 column by a single median condyle or articulating 

 surface. These few characters mark them off dis- 

 tinctly from the other vertebrate animals — the 

 Amphibians, the Fishes, the Birds, and the Mammals. 

 So that the definition of a lizard, which at first 

 sight, perhaps, seemed somewhat formidable, is 

 easily reduced to simple everyday language. The 

 important thing, of course, is to clearly distinguish 

 between a lizard and any other reptile which is not 

 a lizard — a snake for instance ; and when we consider 

 how very like a snake the common slow-worm appears, 



