PALEONTOLOGY. 



is articulated to the vertebral column by a single condyle 

 with several articulating surfaces. 



The bones of the trunk present many modifications in their 

 form and arrangement. In the lizards, the ribs are more 

 numerous than in birds and mammals, and they extend 

 around the abdominal as well as the thoracic organs. In 

 serpents, the sternum is absent, and the ribs are important 

 instruments for locomotion, when added to the ball and 

 socket mode of articulation of the spinal column. Their 

 number is very considerable, amounting in some to 300 pairs. 



In the frogs the converse of this type is found. The ribs 

 are absent, and the sternum is greatly developed, to form 

 with the bones of the shoulder a girdle for the articulation of 

 the anterior extremities. 



In the tortoises, the thoracic and abdominal organs are 

 inclosed in an osseous case, into which likewise the head 

 and extremities can be withdrawn. It is formed of two 

 shields, the dorsal or carapace, and the ventral or plastron, 

 joined together at the sides. The expansion of the ribs forms 

 the carapace, and the elements of the sternum the plastron. 

 This cuirass is externally coated with large scaly plates. 



The extremities present modifications corresponding with 

 their varied modes of life. Thus, in the land tortoises and 

 lizards they are constructed for walking. In the chameleon 

 the bones of the hand are arranged in two divisions for 

 grasping, like a forceps, the branches of the trees in which 

 they live. In those formed for an aquatic life, the extremities 

 are converted into flat oars for swimming. Such is the case 

 in the marine turtles, and in the ancient marine saurians, the 

 ichthyosaurus and plesiosaurus. Those which are destined 

 for an arborial life have the skin of the flanks stretched out 

 upon the six false ribs which extend horizontally outwards 

 to form a parachute for sustaining the lizard whilst leaping 

 from branch to branch. Other extinct forms, the pterodac- 

 tyle, had one finger of the hand enormously developed into 

 a rod for supporting a true wing like the bats. 



In consequence of the arterial blood of reptiles being 

 always mixed with a proportion of venous blood, the tempe- 

 rature of their bodies is nearly that of the surrounding 

 medium. Their organic functions are, therefore, much 

 influenced by changes of temperature. Cold greatly dimi- 



