It is interesting to observe the different way in which this result is attained.

In the mammals the teeth, originally more complex in construction and fewer in number, are converted into efficient grinders by infolding and elongation of the crown of each tooth so as to produce on the wearing surface a complex pattern of enamel ridges with softer dentine or cement intervening, making a series of crests and hollows continually renewed during the wear of the tooth.

In the reptile the teeth, originally simple in construction but more numerous and continually renewed as they wear down and fall out,[15] are banked up in several close packed rows, the enamel borders and softer dentine giving a wearing surface of alternating crests and hollows continually renewed, and reinforced from time to time, by the addition of new rows of teeth to one side, as the first formed rows wear down to the roots.

This is the best illustrated in the _Trachodon_ (see fig.

27); the other groups have not so perfect a mechanism.
A.THE IGUANODONTS: IGUANODON, CAMPTOSAURUS.
_Sub-Order Ornithopoda or Iguanodontia._ In the early days of geology, about the middle of the nineteenth century, bones and footprints of huge extinct reptiles were found in the rocks of the Weald in south-eastern England.