Ch. XVIIL] 



WEALD CLAY. 



U7 



freshwater strata in the delta of the Ganges. But if part of that 

 delta should sink down so as to be covered by the sea, marine forma- 

 tions might begin to accumulate in the same space where freshwater 

 beds had previously been formed ; and yet the Ganges might still 

 pour clown its turbid waters in the same direction, and carry seaward 

 the carcases of the same species of alligator, in which case their 

 bones might be included in marine as well as in subjacent freshwater 

 strata. 



The Iguanodon, first discovered by Dr. Mantell, has left more of its 

 remains in the Weald en strata of the south-eastern counties and Isle 

 of Wight than has any other genus of associated saurians. It was an 

 herbivorous reptile, and regarded by Cuvier as more extraordinary 

 than any with which he was acquainted ; for the teeth, though bear- 

 ing a great analogy, in their general form and crenated edges (see figs. 

 338 a, 338 6), to the modern Iguanas which now frequent the tropi- 



Fig. 



Fig. 338. 



4^ 



a, o. Tooth of Iguanodon Mantelli. 



a. Partially worn tooth of young individual of the safne. 



1). Crown of tooth in adult, worn down. (Mantell.) 



cal woods of America and the West Indies, exhibit many striking 

 and important differences. It appears that they have often been worn 

 bv the process of mastication ; whereas the existing herbivorous rep- 

 tiles clip and gnaw off the vegetable productions on which they feed, 

 but do not chew them. Their teeth frequently present an appearance 

 of having been chipped off, but never, like the fossil teeth of the Igua- 

 nodon, have a flat ground surface (see fig. 339 &), resembling the 

 grinders of herbivorous mammalia. Dr. Mantell computes that the 

 teeth and bones of this species which passed under his examination dur- 

 ing twenty years must have belonged to no less than seventy-one dis- 

 tinct individuals, varying in age and magnitude from the reptile just 

 burst from the egg, to one of which the femur measured twenty-four 



