8o REPTILES 



increasing in size until the whole crown becomes dark brown or 

 blackish, crossed by irregular yellow lines marking the hind 

 borders of the scales. In the allied P. platyrhinus the mark- 

 ings on the hind part of the back are apt to become obsolete 

 in the adult. Another type of variation has been recorded in 

 the Mexican poisonous lizard {Heloderma suspectum), in which 

 the dark rings on the tail and limbs of the young break up into 

 irregular spots in the adult. The lizards of the genus Ger- 

 rhonotus exhibit in a marked degree the more brilliant colouring 

 of the young, and also show in that condition a distinct pattern 

 of alternate light and dark bars which tends to fade out as 

 maturity is attained. The age and sexual differences in colour 

 and pattern in the genera Ameiva and Cnemidop hones have been 

 already noted. 



Perhaps, however, the most remarkable colour-change of 

 this type occurs in an African snake known as Grayia ornata. 

 In the young of the phase described as Grayia ornata furcata 

 the body is black with broad whitish or greyish transverse 

 bars, which split on each side so as to form a series of reversed 

 Ys ; each Y being slightly speckled with black about the 

 middle line. With advancing age the black ground changes to 

 grey or brown ; whereas the Ys show more and more black until 

 they have only white margins, and even these eventually disap- 

 pear. Consequently, the fully adult snake has black markings 

 on a grey ground, in place of the light markings on a black 

 ground characteristic of the young ; in other words, the colour- 

 pattern of the adult is precisely the reverse of that of the 

 young. 



In regard to colour-variation in British lizards the following 

 observations have been published by Dr. G. R. Leighton in his 

 Life-History of British Lizards. 



" Very young specimens of Lacerta vivipara, the common 

 lizard, are nearly black, while the adults are brown and 

 spotted ; the under-parts red or orange in the males with black 

 spots, while in the females these portions ars yellowish. The 

 young of the sand-lizard, L. agi/is, are greyish-brown with 

 spots, whitish underneath ; the adult male is typically green on 

 the sides, the female more brown. In L. viridis, the green 

 lizard, the yellow lateral stripes which are found in the young 

 persist in some of the old females." 



