THE ORNITEOSCELIDA. 227 



many other genera ; but it seems to liave become anchylosed 

 in CompsognathuSj OriiitJiotarsus^ and Euskdosaurus. 



The reptiles belonging to this group are for the most part 

 of very large size, and some of them, as the Iguanodon^ are 

 among the largest of known terrestrial animals. They oc- 

 cur throughout the whole range of the Mesozoic formations, 

 being represented by Thecodooitosaurus^ JPalmosaurus^ Tera- 

 tosaurus^ JPlatmosaurns^ and other genera in the Trias ; by 

 Scelidosaums in the Lias ; by 3fegcdosauriis, Poikiloplenron^ 

 Euskelosaurus^ Hylmosaurus^ Polacanthus^ Acanthopliolis^ 

 Jguanodon^ Iladrosauriis^ Trachodon. and Laelaps in the 

 middle and upper Mesozoic strata. 



There is no evidence that Megaloscmrus^ or Iguanodon^ 

 possessed any dermal armor; but several genera (e.g., Sceli- 

 dosaiirus, Uglceosaurus, and Aca)itho2)holis) had osseous 

 dermal scutes, sometimes produced into prodigious spines. 



The faces of the centra of the vertebras are slightly am- 

 phicoelous, or nearly flat ; but those of the anterior dorsal and 

 cervical regions seem, in some cases, to have been opisthocoe- 

 lous. The sacrum seems to have consisted of at fewest four 

 vertebrae, which in some [Scelidosaurus) are crocodilian, in 

 others {Megalosatcrus) take on a somewhat ornithic character. 

 The caudal region had many and long vertebrae, between 

 which the chevron-bones are attached. The rami of the chev- 

 ron-bones have their vertebral ends united by bone. 



The thoracic vertebral ribs are very strong ; but the sternal 

 ribs and sternum are unknown. However, there is some rea- 

 son to think that the sternum was broad and exjDanded. Ab- 

 dominal dermal ribs are developed in some species, if not 

 in all. 



The structure of the skull seems to have been intermediate, 

 in many respects, between the crocodilian and the lacertilian 

 types. In Iguanodon and Sypsilophodon^ the extremities of 

 the premaxilhie appear to have been edentulous 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 MegalosauTus^ to the broad grinders, wearing 

 down by mutual attrition, of Iguanodon, Their mode of im- 

 plantation varies, but they are not anchylosed to the jaws. 



The scapula is vertically elongated, narrow, and devoid of 

 any acromial process ; the coracoid rounded and without fon* 

 tanelles or processes. 



