ROOM III. TYMPANIC BONE OF THE IGUANODON. 255 



mouth by the large prehensile tongue, which is indicated by 

 the procumbent and inferiorly excavated symphysis. The 

 upper molars are also distinguished by the smaller antero- 

 posterior diameter of the crown by the great prominence of 

 the primary ridge by the breadth of the vertically convex 

 surface of the fang by the width of the lateral facets and 

 by the contraction of the internal or vertically concave sur- 

 face which becomes ridge-like below. 1 



As it is very rarely that a specimen occurs in which the 

 absorption of the fang, from the upward growth and pressure 

 of a new tooth, has not taken place in a greater or lesser de- 

 gree, it is evident that the formation of successional teeth was 

 in constant progress at all periods, as is the case in most 

 saurians. 



We have seen that the internal structure of the teeth is in 

 striking accordance with the external form and mechanical 

 arrangement of the dental organs; for the central body of 

 dentine or tooth-ivory is of a softer and coarser texture than 

 in any known reptiles, and closely resembles that of the 

 large herbivorous mammalians. The peculiar arrangement 

 of substances of different degrees of hardness, must have 

 rendered the teeth in every stage instruments admirably 

 adapted for the trituration and comminution of vegetable 

 substances. The dental pulp became ossified in the old teeth, 

 so that whatever the degree of abrasion, the exposed masti- 

 cating surface was solid. This is seen even in the last 

 stage, when the crown is reduced to a mere plate or disk 

 of dentine. 



TYMPANIC BONE. Wall-case C, see p. 227. As the arti- 

 cular piece which contains the socket of the lower jaw for 

 receiving the inferior head of the os quadratum is unfortu- 

 nately wanting, the mechanism of the articulation of the 

 jaws can only be conjectured; for although a very fine 

 example of the tympanic bone which in reptiles as in birds 

 connects the lower with the upper maxilla is preserved in 

 my former collection, the specimen is not sufficiently perfect 

 to indicate the adjustment of these parts of the maxillary 

 organs. 



1 The distinction between the upper and lower molars was first 

 suggested, and subsequently worked out and established by Dr. Melville. 



