REPTILES. 69 



land and which bring forth living young. These sea- 

 snakes are very poisonous. The rattlesnakes are the best 

 known poisonous forms in the United States. In these 

 the rattle is formed by bits of dry skin, which are not lost 

 at the time when the snake sheds the rest of its covering. 

 In this way a new joint is added to the rattle at each molt, 

 and so the whole becomes an approximate index of age. 



OBDER III. TESTUDIJTATA (Turtles). 



The turtles and tortoises are characterized by their short 

 bodies, enclosed in a bony shell or box ; by the absence of 

 teeth ; and by the union of the quadrate bone with the 

 cranium. The shell, with its two parts, carapace and 

 plastron (p. 56), is composed of an outer layer of horny 

 plates (modified scales) and a deeper bony layer, with 

 which ribs and vertebrae are more or less completely united. 

 Into this protective case the head, tail, and legs may be 

 retracted, and in the box-tortoises a hinge in the plastron 

 allows the closure of the openings. 



Some turtles are vegetarians, others are carnivorous. 

 Some live on land, some in fresh water, and some in the sea. 

 The largest of existing species are the giant land-tortoises 

 of the Galapagos Islands and Mozambique, and the leather- 

 back and the loggerhead turtles of tropical seas. 



Tortoise-shell, before the days of celluloid, was furnished 

 by the dorsal plates of the large tortoise-shell turtle of 

 tropical seas. These plates have the peculiarity that they 

 can be united by heat, so that pieces of any desired size 

 may be obtained. While many turtles are most inoffensive 

 creatures, others, like our snapping- turtles and our soft- 

 shelled turtles, are ferocious, the young snapper showing its 

 temper as soon as it is hatched from the egg. 



