8 CHELONIADA. 
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 
