84 BRITISH LIZARDS 



genus from another. Some of the scales have become 

 much larger than others, probably from fusion of the 

 original small ones. Thus on the top of the head will 

 be found some scales larger than those of the back, 

 and of a different shape. As it is necessary for 

 purposes of accurate description to give these larger 

 scales or head shields definite names, they have been 

 named fr^m the particular positions they fill on the 

 head, e.g. nasal shield. These various names must 

 now be studied. 



THE TERMINOLOGY OF THE HEAD SCALES. 



In describing the head of any reptile we distin- 

 guish three aspects or surfaces. Two of these are 

 visible when the lizard is in the natural attitude on 

 the ground, the third only when lifted off the ground. 

 First of all we have the upper aspect, that of the top 

 of the head, the vertex or dorsal aspect. Secondly, 

 there is the side view of the mouth and eyes, or the 

 lateral aspect. These are the two ordinarily visible. 

 Thirdly, there is the under surface of the head, that of 

 the chin and throat, or the ventral aspect. On each 

 of these surfaces are scales or shields arranged in a 

 definite manner for each species, and named either after 

 their position with regard to other parts of the head, 

 or after some of the cranial bones which they cover. 



These shields vary immensely in different species, 

 some having: more than others. In order to under- 



