308 PETRIFACTIONS AND THEIR TEACHINGS. CHAP. III. 



some physiological deductions from their peculiar osteological 

 characters, we are at once struck with their remarkable de- 

 viation from all known types in the class Reptilia. In the 

 Amblyrhynchi, the most exclusively vegetable feeders of the 

 saurian order, the alveolar process, beset with teeth, is con- 

 tinued round the front of the mouth; the junction of the 

 two rami of the lower jaw at the symphysis presenting no 

 edentulous interval whatever, and the lips not being more 

 produced than in other reptiles ; for these creatures chip off 

 and bruise their food, and cannot grind or masticate it: in 

 the Iguanas, as previously shown, the same character exists. 

 In the carnivorous saurians the teeth are also continued to 

 the symphysial suture on each side. The extinct colossal 

 lizards offer no exception to this rule ; in the acrodont Mosa- 

 saurus of the Chalk, and in the thecodont Megalosaurus of 

 the Oolite and Wealden, the jaws are armed with teeth round 

 the anterior extremity. In short, the edentulous, expanded, 

 scoop-shaped, procumbent, symphysis of the lower jaw of the 

 Iguanodon, has no parallel among either existing or fossil 

 reptiles, and we seek in vain for maxillary organs at all analo- 

 gous, except among the herbivorous mammalia. The nearest 

 approach is to be found in certain Edentata, as for example 

 in the Cholcepus didactylus, or Two-toed Sloth, in which the 

 anterior part of the lower jaw is edentulous and much pro- 

 longed. The correspondence is still closer in the gigantic ex- 

 tinct Mylodon, in which the symphysis resembles the blade oi 

 a spade used by turf-diggers, and has no traces of incisive 

 sockets; and were not this part of the jaw elevated vertically 

 in front, and the two rami confluent, it would present the 

 very counterpart of that of the Iguanodon. 1 



The great size and number of the vascular foramina distri- 

 buted along the outer side of the dentary bone, and beneath 

 the border of the symphysis, in the Iguanodon, and the mag- 

 nitude of the anterior outlets which gave exit to the vessels 



1 In the Mylodon Darwinii the rami of the lower jaw anterior to the 

 teeth are contracted vertically, and converge to a longer and narrowei 

 symphysis, which is inclined forwards at a more open angle with the 

 horizontal ramus, than in the Mylodon robustus, and therefore stil! 

 more nearly approaches that of the Iguanodon. See Professor Owen on 

 the Mylodon. 



