THE ORMTIIOSCELIDA. 227 



many other genera ; but it seems to have become anchylosed 

 in Compsognathus, Ornithotarsus, and Euskelosaurus. 



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 know^n terrestrial animals. They oc- 

 cur throughout the whole range of the Mesozoic formations, 

 being represented by Thecodontosaurus, Paloeosaurus, Tera- 

 tosaurus, Platmosaurus, and other genera in the Trias ; by 

 Seelidosaurus in the Lias ; by Megalosaurus, Poikilopleuron, 

 Euskelosaurus, Hylceosaurus, Polacanthus, Acanthopholis, 

 Iguanodon, Sadrosaurus, Trachodon, and Laelaps in the 

 middle and upper Mesozoic strata. 



There is no evidence that Megalosaurus, or Iguanodon, 

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

 dosaurus, Hylceosaurus, and Acanthopholis) had osseous 

 dermal scutes, sometimes produced into prodigious spines. 



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

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

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

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

 vertebra, which in some {Seelidosaurus) are crocodOian, in 

 others {Megalosaurus) 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 expanded. 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 premaxillse 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 Megalosaurus, 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. 



