208 THE VERTEBRATE SKELETON. 



former meeting in a ventral symphysis. The ilia are very 

 deep vertically and there are no post-pubes. The astragalus 

 is closely applied to the tibia, in front of which it sends an 

 ascending process, sometimes the two bones appear to have 

 been ankylosed together, as in birds. The metatarsals are 

 elongated and the feet digitigrade. 



The Theropoda vary greatly in size, one of the best known 

 genera Compsognathus was about as large as a cat, another, 

 Megalosaurus, perhaps as large as an elephant. Ceratosaurus 

 is the name of a well-known North American form regarded 

 by many authorities as identical with Megalosaurus. 



Suborder (3). ORTHOPODA. 



This suborder includes the most specialised of the Dino- 

 saurs, certain of which resemble the Theropoda in being 

 bipedal. In some of them such as Stegosaurus the exoskeleton 

 is strongly developed, in others such as Iguanodon it is 

 absent. 



The vertebrae are solid and may be opisthocoelous, bi- 

 concave, or flat. The teeth are compressed and serrated, 

 often irregularly, and are frequently not set in distinct sockets. 

 The anterior part of the premaxillae is without teeth, and a 

 toothless pre-dentary or mento-meckelian bone is present. 

 The pre-orbital vacuities are small or absent, and the nares 

 are large and placed far forwards. 



The most characteristic features of the group are found 

 in the pelvis which, except in the Ceratopsia, bears a striking 

 resemblance to that of birds. The ischium and post-pubis 

 are long slender bones directed backwards parallel to one 

 another, and the pre-pubis is also well developed. The ischium 

 has an obturator process. The limb bones are sometimes 

 hollow, sometimes solid. The anterior limbs are much shorter 

 than the posterior, pointing to a bipedal method of progression. 

 The pes is digitigrade or plantigrade, and has three, rarely four, 

 digits. 



