XXIII] SAURIA 585 



the jaws are very powerful, the quadrate remaining immovably 

 clamped by the quadratojugal. The cloacal opening became a 

 longitudinal slit and developed the single median copulatory organ 

 on its front wall. This stock includes the Crocodiles. The Chelonia 

 or Turtles and Tortoises are probably descended from Monapsid 

 Reptiles, of which numerous remains occur side by side with those 

 of Diapsid Reptiles. In Monapsid Reptiles there is only one temporal 

 arch constituted of jugal, quadratojugal, postfrontal and squamosal 

 and one temporal fossa above it, which corresponds to the supero- 

 temporal fossa of Diapsida. Chelonia resemble Crocodilia in the 

 cloaca and copulatory organ. 



FIG. 289. A limbless Lizard, Anguis frag His, the Blind-worm, slightly reduced. 



Order II. Sauria. 



"We now turn to the Sauria. Most people would imagine that 

 the task of distinguishing a Lizard from a Snake was an easy one. 

 But if we were to collect together all the limbless species of 

 Reptiles we should find not only that they differ very much from 

 one another in the structure of the skull and in other points, but 

 that they are more nearly related to different families of Lizards 

 than to one another. There is no doubt that the snake-like forms 

 have been derived from four-limbed reptiles like Lizards, for some of 

 them have rudimentary vestiges of limbs. It is evident then that 

 there must be an advantage in certain situations in getting rid of 

 limbs, and it is further evident that the effect of this advantage 

 has been that not only in one but in many families of Lizards 



