12 INTRODUCTION 



represent the lower eyelid of Lizards. The view as 

 to this homology is derived from our knowledge of 

 various conditions in certain series of Lizards of the 

 families Lacertidae and Scincidae, where we find a 

 transparent disc appearing like a small window in 

 the movable lower eyelid, gradually increasing in 

 size so as to occupy the whole of the lower eyelid, 

 which finally becomes fused with the rudimentary 

 upper lid and loses its mobility. In Ilysia and in 

 most of the Uropeltidas, the transparent disc over 

 the eye is confluent with a thick horny shield of 

 which it occupies the middle. 



The pupil is usually circular or vertical, rarely 

 horizontal. In some forms it is difficult to decide 

 whether it is round or vertically elliptic ; in others, 

 like the Boas and Vipers, for instance, it is decidedly 

 vertical, and contracts to the same extent as a cat's. 

 In some Water-snakes, and in Sea-snakes generally, 

 the round pupil may contract to a mere dot. The con- 

 traction of the pupil is independent on the two sides. 



The snout, or the part of the head anterior to the 

 eyes, may be short or long, rounded or pointed, de- 

 pressed or compressed, sometimes projecting strongly 

 beyond the mouth, turned up at the end, or ter- 

 minating in one (Langaha) or two (Herpcton) long 

 scaly dermal appendages. In some burrowing forms 

 it is provided with a more or less trenchant hori- 

 zontal or vertical edge. When the sides of the snout 

 (loreal region) form an angle with the upper surface, 



