in the number of vertebrae
coossified into a solid sacrum, in the proportions of the ilium and so
on. Various features in the anatomy of the head, shoulder-blades and
hind limbs are equally suggestive of birds, and it seems probable that
the earliest ancestors of the birds were very closely related to the
ancestors of this group of Dinosaurs. But the ancestral birds became
adapted to flying, the ancestral Predentates to terrestrial life, and
in their later development became as widely diversified in form and
habits as the warm-blooded quadrupeds which succeeded them in the Age
of Mammals.

[Illustration: Fig. 25.--Skulls of Iguanodont and Trachodont
Dinosaurs. _Iguanodon_ and _Camptosaurus_ of the Jurassic and
Comanchic; _Kritosaurus_ and _Corythosaurus_ of the Middle
Cretacic (Belly River); _Saurolophus_ of the late Cretacic
(Edmonton); _Trachodon_ of the latest Cretacic (Lance). The
Iguanodon is European, the others North American. All 1/25 natural
size.]

These Beaked Dinosaurs were, so far as we can tell, all vegetarians.
Unlike the birds, they retained their teeth and in some cases
converted them into a grinding apparatus which served the same purpose
as the grinders of herbivorous quadrupeds. 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