THE OKNITHOSCELIDA. 265 



occur throughout the whole range of the Mesozoic forma- 

 tions, being represented by Tliecodontosaurus, Palceosau- 

 nis, Teratosatirus, Platceosaurus, and other genera in the 

 Trias; by Scelidosaurus in the Lias: by Megalosaurus, 

 PoiMlopleuron, Eushelosaurus, Hijloeosaurus, Polaccmthus, 

 Acanilwpliolis, Iguanodon, Sadrosaums, Trachodon, and 

 Laelaps in the middle and upper Mesozoic strata. 



There is no evidence that Megalosaurus, or Iguanodon, 

 possessed any dermal armour; but several genera {e.g., 

 Scelidosaurus, Hijloeosaurus, and Acanthopholis) had osseous 

 dermal scutes, sometimes produced into prodigious spines. 



The faces of the centra of the vertebrae are slightly 

 amphicoelous, or nearly flat ; but those of the anterior dorsal 

 and cervical regions seem, in some cases, to have been opis- 

 thocoelous. The sacrum seems to have consisted of at 

 fewest four vertebrae, which in some {Scelidosaurus) are cro- 

 codilian, in others {Megalosaurus) take on a somewhat or- 

 nithic character. The caudal region had many and long 

 vertebrae, between which the chevron bones are attached. 

 The rami of the chevi'on bones have their vertebral ends 

 united by bone. 



The thoracic vertebral ribs are very strong ; but the 

 sternal ribs and sterntim are unknown. However, there is 

 some reason to think that the sternum was broad and ex- 

 panded. Abdominal dermal ribs are developed in some 

 species, if not in all. 



The structure of the skuU seems to have been interme- 

 diate, in many respects, between the crocodilian and the 

 lacertilian types. In Iguanodon and Hypsiloplwdon, the 

 extremities of the premaxilla3 appear to have been edentu- 

 lous and beak-like ; and the symphysis of the mandible is 

 excavated to receive the beak, almost as in the mandible of 

 a Parrot. 



The teeth vary extremely, from the sharp, recurved, ser- 

 rated fangs of 3Iegalosaurus, to the broad grinders, wearing 

 down by mutiial attrition, of Iguanodon. Their mode of 

 implantation varies, but they are not ankylosed to the jaws. 

 The scapula is veiiically elongated, narrow, and devoid of 



