744 



THE POPULAR SCIENCE MONTHLY. 



And now the transformation is complete, for, when we pass on to 

 the next division of backboned animals, the " reptiles," we hear noth- 

 ing more of gills, nor air taken from the water, nor fins, nor fishes' 

 tails. We shall all allow that the tortoises are the most singular of 

 any. Slow, ponderous creatures, with hard, bony heads, wide-open, 

 expressionless eyes, horny beaks, and thick, clumsy legs, the tortoises 

 seem, at first sight, to be only half alive, as they lumber along, carry- 

 ing their heavy shell, and eating, when they do eat, in a dull, listless 

 kind of way. This sluggishness would certainly be their ruin, in a 

 bustling, greedy world, if it were not for the strong box in which they 

 live. You would hardly guess that the shell of the tortoise is part of 

 his skeleton. But it is so. The arched dome which covers his back 

 is made of his backbone and ribs, and the shelly plates arranged over 

 it are his skin hardened into horny shields, which, in the hawk's-bill 

 turtle, form the tortoise-shell which is peeled off for our use ; while 

 the flat shell under his body is the hardened skin of his belly, and the 

 bones which belong to it. 



J\NOW|. 



It is not without some struggle that the cold-blooded reptiles have 

 held their own in the world, nor is it to be wondered at that only four 



