PROF. OWEN ON THE SKULL OF MEGALOSATTRTJS. 335 



were of the same size, as well as form and structure, there was no 

 ground for predicating distinction of species. 



In the Blenheim specimen I was permitted to expose the germs 

 and portions of the successional teeth concealed in the substance of 

 the mandible *. 



Before entering on the description of the first of the present series 

 of fossils which demonstrates cranial characters not hitherto de- 

 termined, I may premise that existing Saurians show differences in 

 the degree of ossification of the outer wall of the facial part of the 

 skull. 



In Crocodilia it is entire from the relatively small orbit behind 

 to the smaller single nostril in front ; and there is no break in that 

 wall in modern and tertiary species, answering to the antorbital 

 vacuity in Liassic genera ; but this opening, recalling the antorbital 

 nostril of Ichthyosaurus, is very small and is margined by the 

 maxillary, lacrymal, and nasal bones. 



In existing Lacertians much difference is seen in this character ; 

 but in none is the face so completely ossified as in the Crocodiles. 

 The Monitors {Thorictes, Tupinambis) come nearest thereto, the 

 nostril being divided from the orbit by a broad triangular facial 

 plate of the maxillary, supplemented behind by a narrow malo- 

 lacrymal one. In Lacerta the lacrymal enters in larger proportion 

 into the formation of this part of the bony face, and the external 

 nostrils are relatively wider. In Iguana the facial wall dividing 

 the nostril from the orbit is relatively narrower, and the apex of the 

 maxillary process is further removed by a large interposed lacrymal 

 from the nasal bone. But in the Lacertians with a carnivorous 

 dentition (Hydrosaurus, Varanus) the outer bony nostrils are re- 

 markable for their great relative size, especially length; and the 

 maxillary sends upward and backward a long but narrow pointed 

 plate, which, in Varanus bivittatus, crosses in front of a small 

 lacrymal bone to articulate with the prefrontal. 



Here we attain the cranial modification which forms the best 

 guide to the interpretation of the appearances presented by the 

 fossil, the subject of Plate XI. fig. 1, and restored on a Yaranian 

 type in the Cut, p. 340. 



But, before entering on this comparative survey, I may note the 

 corresponding degree of resemblance which the skull of Iguanodon 

 presents to those of the herbivorous and mixed-feeding Lacertians, 

 Iguana and TJiorictes, with correspondingly adaptive shapes of the 

 teeth. In the relative size of the external nostril Iguanodon Foxii 

 resembles Tupinambis more than it does Iguana with the larger nostril ; 

 and the side wall between nostril and orbit is relatively broader and 

 more extensive in Iguana than in either of the blunt- or thick- 

 toothed Lacertians. I may remark also that, as usual in the larger 

 forms, the orbits are relatively smaller than in the dwarfed kinds. 



In all the Lacertians here compared, teeth are developed from the 

 whole {Iguana, Tupinambis), or nearly the whole {Hydrosaurus, 

 Varanus), of the alveolar border of the maxillary ; consequently this 

 * See ' Remarks ' thereon in the volume above cited. 



