In the marine formations are found the fossil remains of the sea-animals of the period; in the coast and delta formations are the remains of those which inhabited the marshes and forests of the coast regions; while the animals of the dryland, of plains and upland, left their remains in the river-plain formations.
[Illustration: Fig.

5 .-- Geologic Cycles and the Land Area of North America (after Schuchert).] These last, however, fragmentary and loose and overlying the rest, were the first to be swept away by erosion during the periods of elevation; and of such formations in the Age of Reptiles very little, if anything, seems to have been preserved to our day.

Consequently we know very little about the upland animals of those times, if as seems very probable, they were more or less different from the animals of the coast-forests and swamps.

The river-plain deposits of the Age of Mammals on the other hand, are still quite extensive, especially those of its later epochs, and afford a fairly complete record in some parts of the continent of the upland fauna of those regions.
_Occurrence of Dinosaur Bones._ 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.