REPTILES. 



317 



chanter. The ceratopsia are best developed in the upper cretaceous of 

 the western U. S., but have also been found in Austria. They were won- 

 derfully protected by their armor and the frontal 

 horns against enemies. Agathaumas (Cera- 

 tops) and Polyonax (Triceratops), from the 

 Laramie beds, are best known. C. ORNI- 

 THOPODA. Digitigrade orthopods without 

 exoskeleton. Vertebrae of neck opisthocoele ; 

 fore legs very short ; prepubes free, postpubes 

 slender and parallel to ischium ; bones of ex- 

 tremities hollow ; .digits with pointed claws. 

 Iguanodon from the upper cretaceous of Eu- 

 rope, and Hadrosaurus (Diclontus} from the 

 green sand of England and America, are best 

 known. Laosaurus, upper Jurassic of Wyo- 

 ming. One species of Igiianodon was 33 feet 

 long:. 



ORDER VII. SQUAMATA (LEPIDO- 

 SAURIA, PLAGIOTREMATA). 



Scaled reptiles with usually procoe- 

 lous vertebrae ; ribs with a single head, 

 no abdominal ribs ; sacrum, when pres- 

 ent, consisting of two vertebrae ; quad- 

 rate free ; supratemporal fossa present 

 or postfrontal arch incomplete ; jugal 

 arch always incomplete ; teeth acrodont saurus ^ after c P e ( see Fi g- 

 or pleurodont ; cerebellum very small, 



optic lobes approximate ; ventricles of heart incompletely sep- 

 arated ; vent a transverse slit ; two hemipenes. 



Lizards and snakes are frequently regarded as constituting 

 two distinct orders ; but in spite of the absence of feet and 

 some other characters, the two groups (together with the ex- 

 tinct pythonomorphs) have so many points in common that the 

 order here recognized is justified. The body is covered with 

 horny epidermal scales, and frequently these are re-enforced by 

 dermal ossifications. In only rare instances are the vertebrae 

 amphiccelous. The nasal apertures in the skull are separate ; 

 the lungs are simple sacs ; limbs, when present, are ambulatory 

 or natatory. 



FIG. 311. Skull of Hadro- 



