TERTIARY SPECTACLES IN REPTILES 457 



inhabiting such an abrasive place as a stone wall. The formation of a 

 milky film under the soon-to-be-shed stratum corneum of the skin, all 

 over the body, can be particularly easily noted in the transparent spec- 

 tacle, and has given rise to the widespread belief that snakes are blind 

 when about to moult. The animal stops feeding, seeks water to soak the 

 loosened comified layer, and is irritable and sluggish; but how much its 

 vision is actually dimmed is a moot point. 



Outside of the snakes, the tertiary spectacle as an adaptation to loco- 

 motor substrates is found only in two turtles (where it is a temporary 

 goggle, like a nictitans) and in a few families of lizards. Though no 



Fig. 155 — The possible ancestor of the permanent tertiary spertacle: the fenestrated lower 

 lid of a desert lizard, the scincid Mabuia vittata. x 6. After Schwarz-Karsten. 



//- upper edge of lower lid; ul- lower edge of upper lid; w- window in lower lid. 



embryological studies have yet been made, it appears to have been 

 formed here from the lower lid also — at least, what seems to be a half- 

 way stage is seen in those deserticolous skinks and lacertids which have 

 clear windows in their lower lids (Fig. 155), as do the turtles mentioned 

 (Emyda granosa and Chelodina longkollis) . When we examine the liz- 

 ards for ecological correlations with the spectacle, we find that essentially 

 two habits seem to have demanded its production: burrowing, and noc- 

 tumality. The former we can readily understand; for whether the lizard 

 remains perpetually under firm ground or has only the problem of loco- 

 motion through shifting sand, poking the head up now and again, the 



