CLASS REPTILIA 401 



of a dorsal carapace and a ventral plastron. Into this shell may be 

 drawn the head and neck, Hmbs, and tail, when the animal is threatened 

 with danger. Although turtles breathe by means of lungs, they may 

 remain under water for a considerable time before needing to come 

 to the surface for air. Since the shell prevents the lungs from being 

 expanded and contracted, air is pumped into them by movements of the 

 neck and feet. Some aquatic turtles also possess thin-walled sacs 

 on each side of the cloaca which may be alternately emptied and filled 

 with water and through the vascular walls of which respiration may take 

 place while the animal is submerged. 



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Fig. 292. — Common snapping turtle, Chchjdra serpentina (Linnaeus). Abundant over 

 an extensive area in the New World, from southern Canada to Ecuador. It is among the 

 largest American fresh-water turtles, grows to a weight of 30 to 40 pounds and is a bold 

 and aggressive turtle. {Photograj)hed and contributed by George E. Hudson.) 



All turtles are oviparous. Their eggs, which are nearly spherical, 

 are covered with a hard, white shell and deposited in nests in the ground. 



America is the richest of all regions in its turtles and tortoises. Giant 

 tortoises found on islands off the west coast of South America and in 

 the Indian Ocean reach a weight of more than 300 pounds and probably 

 attain an age of over four hundred years. They are relics of past ages 

 and owe their survival to the isolation of the islands on which they live. 

 The largest turtle known is a marine leathery turtle which reaches a 

 weight of 1000 pounds or more. 



423. Economic Importance. — Reptiles are economically either injuri- 

 ous or beneficial. Most snakes, being nonvenomous and destroying 

 injurious insects and mammals, are distinctly beneficial, though some 

 do injury by destroying birds and their eggs and young. Other snakes 

 are dangerous to man because of their venom. Generally speaking, 

 lizards are beneficial because of their insect-eating habits; one or two 



