_ Dinosaur bones are found mostly in the
great delta formations, and since those were accumulated chiefly in
the early stages of great continental elevations, it follows that our
acquaintance with Dinosaurs is mostly limited to those living at
certain epochs during the Age of Reptiles. In point of fact so far as
explorations have yet gone in 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.
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