VERTEBll.E AND SKULL OF THE LIZARD. 91 



'character ; jnst as the snake-like eel is compensated by 

 analogous modifications amongst iishes, and the snake- 

 like centipede among insects. 



OSTEOLOGY OF LIZARDS. 



The transition from the ophidian, or snake-like, to the 

 lacertian, or lizard-like reptiles, is very gradual and easy, 

 if we pass from the serpents with fixed jaws and a scapu- 

 lar arch — as, e. g. the slow-worms (anguis) — to the ser- 

 pentiform lizards with mere rudiments of limbs — as, e. g. 

 the pseudopus. The distinction is effected through the 

 establishment of a costal arclj in the trunk, completed by 

 the addition of a haemal spine (sternum) and haemapophy- 

 ses (sternal ribs) to the pleurapophyses or vertebral ribs, 

 which are alone ossified in ophidia. 



The vertebrae of the trunk have the same procoelian 

 character, i. e. with the cup anterior and the ball behind ; 

 the latter being usually less prominent, more oblique, and 

 more transversely oval than in serpents. The vertebrae 

 also are commonly larger, and always fewer in number 

 than in the typical ophidia. The ribs do not begin to be 

 developed so near the head in lizards. Not only the atlas 

 and dentata, but sometimes, as in the monitor {yaranns)^ 

 the four following vertebrae are devoid of pleurapophyses ; 

 and when these first appear they are short, and sometimes 

 (as in cyclodas) expanded at their extremities. They ra- 

 pidly elongate in succeeding vertebras, and usually at the 

 ninth from the head {cychdus^ iguana)^ or tenth {yaranus\ 

 they are joined through the medium of ossified haemopo- 

 physes to the sternum; two {varanus\ three {chameleo, 

 iguana\ or four {cyclodus\ following vertebree are simi- 



