8 CHELONIAD^E. 



tudinata have the back and sternum covered with horny 

 plates, or scales, it is almost exclusively those of the present 

 species which are applicable to the purposes just named. 

 The thirteen plates with which the whole upper part of the 

 shell is covered, are in fact much thicker and stronger, as 

 well as more beautifully clouded in colour, than those of 

 any other species ; and as they lie one over the other like 

 the tiles of a house, so that at least one-third of each 

 overlaps the one behind it, they are much larger in com- 

 parison with the size of the individual. The scales, or 

 plates, are in the first place separated by the application of 

 heat. They are sold to the manufacturers in the rough 

 state, in which they are uneven, fragile, opaque, and dirty; 

 and it is the first object of the artificer to obviate these 

 defects. The uneven surface, the irregular curvature, the 

 unequal thickness of different parts, have all to be cor- 

 rected ; and not only can these objects be readily effected, 

 but the substance can be rendered ductile, compressible, 

 capable of receiving any impression, of being carved, 

 moulded and polished, and even extended by soldering 

 pieces together by means of their own substance reduced 

 to powder. The whole of these processes are performed 

 by means of heat. 



The uneven curvature is first of all to be removed, and 

 the plate rendered perfectly flat. This is effected by im- 

 mersing it in hot water, and then allowing it to cool under 

 heavy pressure between smooth blocks of wood, or metallic 

 plates. The surface is then rendered smooth, and the 

 thickness equal, by scraping and filing away the rough and 

 prominent parts. In this way each plate receives an equal 

 and smooth surface. But it is in many cases desirable to 

 employ larger pieces than can be obtained from single 

 plates, and two pieces are then united together in the 



