724 EVOLUTION OF ANIMALS Part V 



Order Chelonia — Turtles 



Turtles can be instantly distinguished from all other animals by the shell, a 

 fortress so large in many of them that they can withdraw into it, head, legs, 

 and tail (Fig. 35.12). The order consists of over 200 species that breathe air 

 and lay their eggs on land with some, such as the sea turtles, that spend most of 

 their Uves in water. In general usage, chelonians are called turtles or tortoises 

 with little regard for meanings. The most common three types are: 



Turtles — Semiaquatic in fresh or salt water, e.g., painted and loggerhead turtles 



(Fig. 35.12). 

 Tortoises — Mainly or entirely land dwellers, e.g., wood turtles. 

 Terrapins — Edible with market value, e.g., diamondback terrapin. 



In Britain, tortoise is applied to land and fresh-water species and turtle to 

 marine ones. 



Ancestry. In the early part of the Age of Reptiles certain ones developed 

 horny, toothless beaks and bony casings about the body. Their descendants are 

 the turtles of today. 



Shell. This consists of an upper carapace and lower plastron united on each 



Fig. 35.12. Sea turtles probably originated from ancient marsh-inhabiting an- 

 cestors. They live in the warmer seas encircling the globe. Atlantic green turtle 

 {Chelonia mydas mydas). For the food market the most valuable reptiles in the 

 world, they have been exterminated from many areas by hunting them in the sea, 

 and collecting their eggs on land. They are still a staple food in some Caribbean 

 ports and a delicacy in large American and European cities. The Pacific green 

 turtle is very similar to the Atlantic species. The weights of green turtles now cap- 

 tured are from 25 to 200 pounds, formerly 500 pounds was common. (Photo- 

 graph by Isabelle Hunt Conant.) 



