+ 



62 THE COMMON FROG. [chap. 



of any of these appendages. Allied forms, however^ 

 present us with examples of some interesting epi- 

 dermal conditions. Thus in old male Toads, in Dac- 

 tylethra and in one of the Japanese efts, the epi- 

 dermis of some of the finger-tips becomes hardened 

 and horny, in other words we begin to meet with 

 incipient '* nails." *' Incipient" because, in ascending 

 from the lowest vertebrates, " nails " are first met with 

 in the Frog's class, and these only very rarely and in 

 an imperfectly developed condition. 



As has been mentioned, in two kinds of Frog 

 (C^.ratophrys and Ephippifer) the skin of the back is 

 furnished with bony plates. These are found in the 

 deeper layer or dermis, and are therefore " scutes." 



'.-ZS 



Fig. 28.— Diagram of a vertical section of both Carapace and Plastron of a Tortoise, 

 made transversely to the long axis of the skeleton, c. vertebral cent'-um : ns, 

 neural spine which expands above into a median dorsal scute ; r, rib wh'cti 

 forms one mass with a lateral scUe and terminates at a marginal plate ; ic, inter- 

 clavicular scute ; hp, hyo-sternal scute. 



The remarkable circumstance, however, is that we 

 have here a lower stage (as it were an incipient con- 

 dition) of that more developed dermal skeleton which 

 exists in tortoises and turtles. In most of these 

 reptiles both the back and the belly are protected by 

 bony plates which adjoin one another, and together 

 form' a solid box in which the body is enclosed. 



