EPIDERMAL STRUCTURES. 93 



external surface of the body, but usually with growth the ante- 

 rior end of each extends beneath the posterior margin of the 

 scale in front, so that as a result the scales come to lie in dermal 

 pockets oblique to the surface. In some cases (South American 

 siluroids, many plectognaths, etc.) the scales may fuse into a 

 firm dermal armor enclosing the body, while in many fossil gan- 

 oids this external 

 skeleton was 

 highly developed; "" 



the parts uniting 



in Some instances FlG . 99 . Position of scales (black) in skin of 



into large armor teleost. Z>, derma \ E, epidermis. 



plates. 



All existing amphibia except some caecilians are without 

 scales. 1 In the latter group and in many fossil amphibia they are 

 (or were) well developed. In caecilians the scales are dermal, 

 and lie in the rings which encircle the body. In the stegoceph- 

 alans these plates were in some cases confined to the ventral 

 surface, in some they covered the entire body. 



In the reptilia of all groups forms are found with a well- 

 developed dermal skeleton of bony plates, the plates in Stegosau- 

 rns (one of the extinct dinosaurs) being nearly two feet across. 

 In recent forms similar but smaller dermal bones occur in alli- 

 gators and many lizards, and reach their extreme in the turtles, 

 where these bony plates unite to form a bony box, composed of 

 an upper carapace and a lower plastron, enclosing the trunk. 

 This shell becomes firmly united with the true skeleton, and to 

 a certain extent replaces it in some species, the vertebrae and 

 parts of the ribs being correspondingly reduced. 



Besides this bony skeleton reptiles are also provided with 

 scales, in the formation of which a papilla of the derma is formed, 

 but the scales themselves arise from cornifications of the outer 

 epidermal cells. This horny envelope is periodically moulted by 

 all reptiles except turtles. It may come away piecemeal, or 

 again, as in snakes and lizards, as a continuous whole ; the pro- 

 cess of separation being aided by the formation of hair-like 

 processes developed from the deeper cells which lift up those 



1 Some tropical toads have bony plates beneath the skin of the back. 



