CHELONIA. 



593 



Order CHELONIA. Tortoises and Turtles. 



GENERAL CHARACTERS. The body is compact, encased in 

 bone, broad in the region of the trunk. There is a dorsal and 

 a ventral shield, within the shelter of which the head and neck, 

 tail and limbs, can be more or less retracted. 



The dorsal carapace is usually formed from (a) the 



flattened neural spines (plus dermal bones] \ (b) expanded and 



more or less coalesced ribs (plus dermal bones] ; (c) a series of 



dermal marginal bones around the outer edge. In the Athecce 



the dorsal vertebrce and ribs are not fused to the dermal plates 



which form the carapace. The ventral shield or plastron is 



formed of nine or so dermal bones. There is no sternum. 



FIG. 288. External appearance of tortoise. 



Overlapping, but not corresponding to the bony plates, there 

 are] except in Trionychia and Athectz] epidermic horny plates 

 of "" tortoise shell" which, though very hard, are not without 

 sensitiveness, numerous nerves ending upon them. 



The quadrate is immovably united with the skull. There 

 is only a lower temporal arcade. 



The jaws are covered by a horny sheath, and are without 

 teeth, though hints of these have been seen in some embryos. 

 There is a single anterior nasal opening. 



The scapular arch is internal to the ribs. The limbs are 

 pentadactyl, often in the form of paddles. 



The average life of Chelonians is sluggish. Perhaps this is 

 in part due to the way in which the ribs are lost in the 

 carapace, for this must tend to make respiration less active. 

 The lungs are divided into a number of compartments. 



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