REPTILIA. DINOSAURIA. 209 



The suborder Orthopoda may be further subdivided into 

 three sections : 



A. STEGOSAURIA. 



A dermal exoskeleton is strongly developed. The verte- 

 bral centra are flat or biconcave, and neither they nor the 

 limb bones are hollowed out by internal cavities. The limbs 

 are plantigrade, the anterior ones short, the posterior ones 

 very large and strong. The post-pubis is well developed ; 



e.g. Stegosaurus from the Upper Jurassic of Colorado. 



B. CERATOPSIA. 



There is sometimes a well-developed dermal exoskeleton 

 formed of small granules and plates of bone. The bones are 

 solid, and the vertebral centra flat. The cranium bears a 

 pair of enormous pointed frontal horns, and the parietal is 

 greatly expanded and elevated behind, forming with the 

 squamosals a shield which overhangs the anterior cervical 

 vertebrae. The premaxillae are united, and in front of them 

 is a pointed' beak-like bone which bites upon a toothless 

 predentary ossification of the mandible. The teeth have two 

 roots. The anterior limbs are but little shorter than the 

 posterior ones. There is no post-pubis ; 



e.g. Polyonax from the uppermost Cretaceous of Montana. 



C. ORNITHOPODA 1 . 



There is no dermal exoskeleton. The cervical vertebrae 

 are opisthocoelous, and so are sometimes the thoracic. The 

 limb bones are hollow and the anterior limbs are much shorter 

 than the posterior ones. The feet are digitigrade and pro- 

 vided with long pointed claws. The post-pubis is long and 

 slender and directed back parallel to the ischium ; 



e.g. Iguanodon from the European Cretaceous. 



1 See 0. C. Marsh, Amer. J. Set. (3), vol. 48, 1894, p. 85. 

 R. 14 



