d to the
lizard-like Sphenodon of New Zealand, the latter having its nearest
allies in the same group of reptiles--Rhyncocephala, other forms of
which occur in the Trias. In this last-named formation the earliest
crocodiles--Phytosaurus (Belodon) and Stagonolepis occur, as well as the
earliest tortoises--Chelytherium, Proganochelys, and Psephoderma.[195]
Fossil serpents have been first found in the Cretaceous formation, but
the conditions for the preservation of these forms have evidently been
unfavourable, and the record is correspondingly incomplete. The marine
Plesiosauri and Ichthyosauri, the flying Pterodactyles, the terrestrial
Iguanodon of Europe, and the huge Atlantosaurus of Colorado--the largest
land animal that has ever lived upon the earth[196]--all belong to
special developments of the reptilian type which flourished during the
Secondary epoch, and then became extinct.

Birds are among the rarest of fossils, due, no doubt, to their aerial
habits removing them from the ordinary dangers of flood, bog, or ice
which overwhelm mammals and reptiles, and also to their small specific
gravity which keeps them floating on the surface of water till devoured.
Their remains were long confined to Tertiary deposits, where many living
genera and a few extinct forms have been found. The only birds yet known
from the older rocks are the toothed birds (Odontornithes) of the
Cretaceous beds of the United States, belonging to two distinct families
and many genera; a penguin-like form (Enaliornis) from the Upper
Greensand of Cambridge; and the well-known long-tailed Archaeopteryx
from the Upper Oolite of Bavaria. The record is thus imperfect and
fragmentary in the extreme; but it yet shows us, in the few birds
discovered in the older rocks, more primitive and generalised types,
while the Tertiary birds had already become specialised like those
living, and had lost both the teeth and the long vertebral tail, which
indicate reptilian affinities in the earlier Mammalia have been found,
as already stated, as far back as the Trias formation, in Europe in the
United States and in South Africa, all being very small, and belonging
either to the Marsupial order, or to some still lower and more
generalised type, out of which both Marsupials and Insectivora were
developed. Other allied forms have been found in the Lower and Upper
Oolite both of Europe and the United States. But there is then a great
gap in the whole Cretaceous formation, f