348 SYSTEMATIC ZOOLOGY. 



duced by certain glands in the skin still others by their 

 poison-glands. Most of the snakes are terrestrial, but 

 some, like our water-snakes, take to the water, while in 

 the Indian Ocean are found truly aquatic snakes, which 

 never go on 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 approxi- 

 mate index of age. 



ORDER HI.--TESTUDINATA (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, a dorsal 

 carapace and a ventral plastron^ 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 com- 

 pletely united. Into this protective case the head, tail, 

 and legs may usually 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 



