FEOM THE KIMMERIDGE CLAY. 439 



more closely resemble the teeth in the dentary piece of a mandible 

 obtained from near the horizon of the Purbeck cinder-bed at 

 Swan age, figured by Prof. Owen in Foss. Eept. AVealden and Pur- 

 beck Eormations, Suppl. v. pi. i. (1874). 



Several broken teeth show that both upper and lower teeth have a 

 large pulp-cavity (PI. XYIII. fig. 8). It is now filled with calcite, 

 which, by its whiteness, contrasts strongly with the dark dental tissues. 

 The cavity passes far into the crown, towards the base of which it is 

 widest ; and it contracts greatly towards the end of the fang, which 

 it pierces as a minute inconspicuous pore. The fang in both jaws 

 tapers to a point. I particularly mention this because, though 

 known and described already, it has been lately asserted that such 

 tapering ending of the fang, with accompanying reduction of its pulp- 

 cavity almost to its obliteration, is unusual in Iguanodon Mantelli. 



Succession. — The replacement of older teeth as their crowns are 

 worn out, by germs developed in reserve cavities lying nearer the inner 

 surface of the jaw, is more completely illustrated by these Cumnor 

 fossils than by any which had previously come under my notice. Pour 

 successional phases of upper and lower teeth are apparent. In the 

 maxilla, I. 2, may be seen : — 1, stumps ready to be shed, as that 

 marked ISTo. 5, where the cingulum is 5*5 to 6 millims. beyond the 

 outer alveolar border ; 2, teeth the crowns of which were in full 

 use, as that marked No. 4, where the entire crown has emerged, the 

 cingulum lying nearly in the plane of the alveolar margin ; 3, germ^ 

 crowns, which have emerged only J or -i-, and had not come into 

 use, as Nos. 1, 3, 10, 12, 14 ; 4, quite small germ-crowns, which 

 only just peep over the inner parapet. In these four phases the 

 younger lie inside the older teeth, alternating with them, the arrange- 

 ment being such that, when one of the oldest teeth is ready to fall 

 out, a fully formed successional tooth moves outwards into its place. 



The fragment of lower jaw, I. 3 (fig. 6), illustrates for the lower 

 teeth, even more beautifully, the same four phases, the same grouping 

 and progression. Nos. 1, 3, 5, are teeth nearly worn out ; Nos. 2, 4, 6, 

 teeth in full use ; Kos. 7, 8, 9, crowns scarcely half emerged ; and 

 Wos. 10, 11, minute germs only just visible above the inner parapet. 



An idea of the size of the teeth may be gleaned from the follow- 

 ing measurements. The maxillary piece, I. 2, in a space of 75 

 millims, contains the sockets of an outer series of 9 teeth. The 

 breadth (^. e. antero-posterior dimension) of a fully formed upper 

 crown is 9'5 millims. ; the length of a fang (I. 5) is 20-5 millims. 

 The dentary part of the left mandibular ramus, I. 4, in the space 

 of 94 millims., has the sockets of an outer series of 12 teeth ; and 

 the fragment of the right ramus, I. 3, contains sockets of six outer 

 teeth in the space of 51 millims. The greatest breadth of the 

 largest lower-tooth crown is 12*5 millims., that of other crowns 

 varying between 10 and 11*5 millims. 



The head of this Cumnor Iguanodon, so far as may be inferred 

 from the pieces recovered, may have been about 20 cm. long, a 

 size which may be considered moderate. In its general form it was 



