uld have seemed, until lately, quite certain that these
tracks could only have been formed by Birds. It has, however,
been shown that the Deinosaurian Reptiles possess, in some cases
at any rate, some singularly bird-like characters, amongst which
is the fact that the animal possessed the power of walking,
temporarily at least, on its hind-legs, which were much longer and
stronger than the fore-limbs, and which were sometimes furnished
with no more than three toes. As the bones and teeth of Deinosaurs
have been found in the Triassic deposits of North America, it
may be regarded as certain that _some_ of the bipedal tracks
originally ascribed to Birds must have really been produced by
these Reptiles. It seems at the same time almost a certainty
that others of the three-toed impressions of the Connecticut
sandstones were in truth produced by Birds, since it is doubtful
if the bipedal mode of progression was more than an occasional
thing amongst the Deinosaurs, and the greater number of the many
known tracks exhibit no impressions of fore-feet. Upon the whole,
therefore, we may, with much probability, conclude that the great
class of Birds (_Aves_) was in existence in the Triassic period.
If this be so, not only must there have been quite a number of
different forms, but some of them must have been of very large
size. Thus the largest footprints hitherto discovered in the
Connecticut sandstones are 22 inches long and 12 inches wide,
with a proportionate length of stride. These measurements indicate
a foot four times as large as that of the African Ostrich; and the
animal which produced them--whether a Bird or a Deinosaur--must
have been of colossal dimensions.

[Illustration: Fig. 156.--Lower jaw of _Dromatherium sylvestre_.
Trias, North Carolina. (After Emmons.)]

[Illustration: Fig. 157.--a, Molar tooth of _Micro estes antiquus_,
magnified; b, Crown of the same, magnified still further. Trias,
Germany.]

[Illustration: Fig. 158.--The Banded Ant-eater (_Myrmecobius
fasciatus_) of Australia.]

Finally, the Trias completes the tale of the great classes of the
Vertebrate sub-kingdom by presenting us with remains of the first
known of the true Quadrupeds or _Mammalia_. These are at present only
known by their teeth, or, in one instance, by one of the halves of
the lower jaw; and these indicate minute Quadrupeds, which present
greater affinities with the little Banded Anteater (_Myrmecobius
fasciatus_, fig. 158) of Aust