102 ANIMAL LIFE PAST AND PRESENT. 



underlying bones at the lines of junction with one 

 another. Now it is these horny shields which form 

 the " tortoise-shell " of commerce ; in the land-tortoises 

 they are, indeed, very thin, and of no economical value, 

 but in one of the marine turtles, known as the Hawks- 

 bill, they become greatly thickened, and furnish the 

 well-known "tortoise-shell," so remarkable for its 

 translucency and the beautiful coloration formed by 

 the mottled blending of a full reddish brown with a 

 lemon yellow. It will be noticed from Fig. 27 that in 

 the land-tortoises these horny shields have their edges 

 in apposition ; but in the young of the Hawksbill 

 turtle they overlap one another like the slates on a 

 roof, although in the adult they become united by their 

 edges in the same manner as in the tortoises. 



Turning once more to Fig. 27, it will be seen, as 

 we have already incidentally mentioned, that the shell 

 consists of an upper vaulted portion covering the sides 

 and back, which is technically known as the carapace ; 

 and also of a flattened lower plate protecting the chest 

 and abdomen, to which the term plastron, or breast- 

 plate, is applied. The carapace and the plastron are 

 usually connected together by a comparatively short 

 bony bridge, at the extremities of which are the 

 notches for the fore and hind limbs, as is well shown 

 in Fig. 27. In the land- tortoises this union between 

 the carapace and plastron is complete; but in the 

 marine-turtles, and also in some freshwater tortoises, 

 there is no such bony union between the upper and 

 lower portions of the shell. The characters of the 



