124 REPORT — 1841. 



that the thick body of the tooth might be worn away in a more regularly ob- 

 lique plane, the dentine is rendered softer as it recedes from the enameled 

 edge by the simple contrivance of arresting the calcifying process along cer- 

 tain tracts of the inner wall of the tooth. When attrition has at length ex- 

 hausted the enamel, and the tooth is limited to its function as a grinder, a 

 third substance has been prepared in the ossified remnant of the pulp to add 

 to the efficiency of the dental instrument in its final capacity. And if the 

 following reflections were natural and just after a review of the external cha- 

 racters of the dental organs of the Iguanodon, their truth and beauty become 

 still more manifest as our knowledge of their subject becomes more particu- 

 lar and exact : — 



" In this curious piece of animal mechanism we find a varied adjustment of 

 all parts and'proportions of the tooth, to the exercise of peculiar functions, 

 attended by compensations adapted to shifting conditions of the instrument, 

 during different stages of its consumption. And we must estimate the works 

 of nature by a different standard from that which we apply to the productions 

 of human art, if we can view such examples of mechanical contrivance, united 

 with so much economy of expenditure, and with such anticipated adaptations 

 to varying conditions in their application, without feeling a profound convic- 

 tion that all this adjustment has resulted from design and high intelligence." 

 — Buckland's Bridgewater Treatise, vol. i. p. 249. 



Head. — Two fragments of jaw with alveoli, in the Mantellian Collection, 

 are referred by its founder to the Iguanodon : in neither of them, unfortu- 

 nately, is a tooth with the characteristic crown preserved : the size of these 

 specimens proves them to have belonged, if to this genus, then to young in- 

 dividuals. The smaller fragment is described in this Report under the head 

 of Hylceosaurus, on account of the cylindrical, equal, and straight form of the 

 remaining fangs. These parts correspond with the fangs of the teeth which I 

 suppose to belong to the Hylaosaurus, rather than with those of the Igua- 

 nodon, which are angular, curved, taper towards a point, and support crowns 

 so expanded, as to require greater intervals between their fangs than in the 

 fossil. It is just possible that these differences may depend on age*. 



Tympanic hone. — A reptile with vertebrae and ribs resembling in their chief 

 characters those of the ccelospondylian Crocodiles, and with distinctive pecu- 

 liarities, in which the Lacertians by no means participate, might reasonably be 

 conjectured to resemble the Crocodiles in the form of the tympanic bone ; and 

 if the reptile in question used its teeth for masticating hard vegetable sub- 

 stances, we might with more reason expect that the bony pillar supporting the 

 lower jaw should be firmly and immoveably fixed through its whole length, like 

 the tympanic bone of the Crocodilians, and not be loosely suspended to the skull 

 by a single extremity, as in the Iguana and other Lacertians. A very remarkable 

 bone discovered in the Tilgate strata, figured by Dr. Mantell in the ' Geology 

 of the South-east of England,' pi. ii. fig. 5, the resemblance of which to the 

 ' OS quadratum,' or tympanic bone of birds, was first suggested by Dr. Hodg- 

 kin, is assigned to the Iguanodon by Dr. Mantell. He accurately describes 

 it " as forming a thick pillar or column, which is contracted in the middle, and 

 terminates at both extremities in an elliptical and nearly flat surface." In the 

 Iguana and other reptiles the lower end of the tympanic bone is terminated 

 by a convex trochlea, which is received into a corresponding cavity in the 

 lower jaw. Is the modification of the bone in question, assuming it to belong 



* In the Monitor-lizards of the modern genera Thorictes and Crocodilurus, the posterior 

 teeth in the young indhiduals have more or less compressed and tri-cuspidate crowns, but 

 in the old animals they have round obtuse crowns, adapted for true mastication. Some mo- 

 dification analogous to this may take place in the Iguanodon. 



