this country, the Dinosaur fauna of the
close of the Jurassic and beginning of the Comanchic and that of the
later Cretacic are the only ones we know much about. The immense
interval of time that preceded, and the no less vast stretch of time
that separated them, is represented in the record of Dinosaur history
by a multitude of tracks and a few imperfect skeletons assigned to the
close of the Triassic period, and by a few fragments from formations
which may be intermediate in age between the Jurassic-Comanchic and
the late Cretacic. Consequently we cannot expect to trace among the
Dinosaurs, the gradual evolution of different races, as we can do
among the quadrupeds of the Age of Mammals.

_Imperfection of the Geologic Record._ The Age of Mammals in North
America presents a moving picture of the successive stages in the
evolution of modern quadrupeds; the Age of Reptiles shows (broadly
considered) two photographs representing the land vertebrates of two
long distant periods, as remote in time from each other as the later
one is remote from the present day. Of the earlier stages in the
evolution of the Dinosaurs there are but a few imperfect sketches in
this country; in Europe the picture is more complete. In the course of
time, as exploration progresses, we shall no doubt recover more
complete records. But probably we shall never have so complete a
history of the terrestrial life of the Age of Reptiles as we have of
the Age of Mammals. The records are defective, a large part of them
destroyed or forever inaccessible.

CHAPTER III.

KINDS OF DINOSAURS.

COMMON CHARACTERS AND DIFFERENCES BETWEEN THE VARIOUS GROUPS.

In the preceding chapter we have attempted to point out the place in
nature that the Dinosaurs occupied and the conditions under which they
lived. They were the dominant land animals of their time, just as the
quadrupeds were during the Age of Mammals. Their sway endured for a
long era, estimated at nine millions of years, and about three times
as long as the period which has elapsed since their disappearance.
They survived vast changes in geography and climate, and became
extinct through a combination of causes not fully understood as yet;
probably the great changes in physical conditions at the end of the
Cretacic period, and the development of mammals and birds, more
intelligent, more active, and better adapted to the new conditions of
life, were the most important factors in their extinctio