THE LIZARDS, SNAKES, AND OTHER REPTILES. 193 



the outgrowth of the spinous processes and ribs of the tho- 

 racic vertebrae, to which are soldered broad, thin, bony 

 plates. Above these are usually thin horny epidermal or 

 skin-plates, forming the tortoise-shell of commerce. The 

 ventral shield or plastron consists of two rows of bony 

 plates covered by skin-plates. 



Turtles are invariably toothless, but the absence of teeth 

 is made up by sharp-cutting horny plates, so that most 

 turtles can bite savagely. On the whole, however, turtles 

 require bat little food, and take long fasts. 



Moreover, they breathe very slowly; they have to, as the 

 chest cannot heave when the air is drawn in, since it is 

 solidly fastened to the carapace. In breathing the air is 

 gently drawn in through the nostrils, and then very slowly 

 passes out again. There is thus a harmony between their 

 slow movements and slow respiration; while the heart is not 

 highly developed, compared with a crocodile's. There are 

 about forty species of turtles in America north of Mexico. 

 The lower forms of turtles are the marine species. Such is 

 the great sea-turtle {Sphargis coriacea) of the Atlantic and 

 Mediterranean, which is the largest of all existing turtles, 

 and is sometimes eight feet long, weighing from eight hun- 

 dred to twelve hundred pounds. The green turtle of the 

 West Indies weighs from two hundred to three hundred 

 pounds, and is used for making delicious soups and steaks, 

 being caught at night when laying its eggs on sandy shores. 

 The sea-turtles have large, flat, broad flippers or fin-like 

 limbs, while in the pond- and river-turtles the feet are 

 webbed and the toes distinct. 



The turtles lay their eggs in sand on the shores of ponds 

 and rivers. In the Middle and New England States nearly 

 all the turtles lay their eggs on or about June 10th, the 

 eggs being hatched late in the summer. Turtles do not lay 

 eggs until from eleven to thirteen years old. 



The land-tortoises, as probably all turtles, are long-lived, 

 and often reach a great age. White, in his " l^atural His- 

 13 



