20 THE STORY OF REPTILE LIFE. 



of bony plates, which, like the horny covering 

 shields, are also symmetrically disposed. 



Generally, in armour-clad animals where the 

 protecting covering is made up of horny shields 

 covering bony plates, the horny outer layer 

 exactly corresponds to the bony plate beneath 

 it. This is not the case with the tortoises and 

 turtles, or, to use a more comprehensive term, 

 embracing all the members of this order, the 

 Chelonians. 



In the specimen from which this figure was 

 drawn, the horny shields in question were removed 

 from one side of the carapace, as this shell is 

 called, so that their relationship one to another 

 and to the bony elements beneath them, can be 

 readily made out. If the exposed portion of the 

 underlying bony shell be examined two sets of 

 patterns can be made out on its surface. One is 

 formed by shallow grooves, which represent the 

 impress of the edges of the horny shields, the 

 other by a series of curiously ziz-zag lines. These 

 represent the sutures or rough jagged edges of 

 distinct bones, which have a very remarkable 

 history, inasmuch as one set those which may 

 be seen running down the middle of the back 

 are formed by table-like expansions of what are 

 known as the " spines " of the vertebrae, or back- 

 bone, and therefore belong to the skeleton ; whilst 

 the remainder, those which take the form of bands, 

 running at right angles to the backbone, and those 

 which form the margin of the shell, are derived 

 from bones originally embedded in the skin. They 

 are this, and something more. 



Since the day when they formed nodules of 



