COLORATION AND ITS INTERPRETATION 87 



which space admits of allusion occurs in the sea-snakes, 

 forming the subfamily Hydrophiuue of the Colubridce. As 

 mentioned above, these snakes are noticeable for their com- 

 pressed form, at least in the region of the tail. The majority 

 are coloured almost exactly like mackerel ; that is to say, the 

 back is dark blue, from which descend bars of the same colour, 

 separated by whitish interspaces of similar width ; while the 

 under surface is of the same silvery hue as these light inter- 

 spaces. Obviously, as in the case of mackerel, the mottled 

 blue of the upper parts harmonises with the rippled surface of 

 the ocean and thus renders these snakes invisible from above ; 

 while their light under-parts when projected against the light of 

 the sky render them equally inconspicuous to enemies from 

 below. In one large species the dark upper surface is orange- 

 brown instead of blue ; the object of this departure from the 

 normal type of coloration being at present unknown. 



Tree-geckos in general, by their mottled hues of grey, 

 white, and black, present a marked protective resemblance to 

 the bark of forest trees ; but the resemblance is carried to its 

 highest degree in one variety of a Malagasy species known 

 as the lichen bark -gecko, Uroplates fimbriates lichenium (see 

 PI. V.). In this species, to which allusion will again be 

 made, the upper surface is dotted with irregular whitish lichen- 

 like markings upon a slate-coloured ground, so that the resemb- 

 lance to lichen-coloured bark must be almost perfect when the 

 creature is at rest with its flattened head, body, and tail closely 

 pressed to the bough. Indeed the only part of the reptile that 

 would appear in the least conspicuous would be its large and 

 bloodshot eyes, and even these may perhaps assimilate in nature 

 to the end of a recently broken twig. A few geckos, such as 

 Naultinus elegans and TJielsuma madagascarieusis, are coloured 

 green to resemble foliage, the former being wholly of this 

 tint, while the latter has the hind part of the back spotted with 

 red. 



As already mentioned, a number of ground-lizards are 

 coloured slaty grey to harmonise with the prevailing hues of 

 the rocks or soil on which they habitually dwell ; but two in- 

 stances may be cited where this protective coloration is speci- 

 ally noticeable. These are the two lizards commonly known 

 under the names of the horned toad (Phrynosoma cornutum) 



