6i 4 



REPTILIA. 



from (a) the flattened neural spines (plus dermal scutes) ; 



(b) expanded and more or less coalesced ribs (plus costal 

 dermal scutes) ; (c) a series of dermal marginal scutes around 

 the outer edge. In the Atheccz the dorsal vertebra 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. 



Overlapping, but not corresponding to the bony plates, there 

 are (except in Trionychia and Atheccz) 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 oj 

 these have been seen in some em- 

 bryos. There is a single anterior 

 nasal opening. The scapular arch 

 is internal to the ribs. The limbs 

 are pentadactyl, but often in ths 

 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. 



The cloacal aperture is usually 

 longitudinal, never transverse ; the 

 copulatory organ is unpaired. 

 The eggs have firm, usually calcareous, 



FlG. 331. Carapace of 

 tortoise. 



The dark contours are those of 

 the bony pieces ; the lighter 

 contours are those of the scales 

 which have been removed. 



All are oviparous, 

 shells. 



Some Peculiarities in the Skeleton of Chelonia 



The (10) dorsal vertebrae are without transverse or articular processes, 

 and along with the ribs are for the most part immovably fused in the 

 carapace. The tail and neck are the only flexible regions. There are 

 two sacral vertebrae. 



The greater part of the dorsal shield is due to a coalescence of eight 

 ribs with eight costal plates derived from the dermis. 



