and the Horse, the Hippopotamus and the Pig, the Whale and the Seal, the Reptiles and Birds, the Ganoid fishes and Batrachia, etc. Not only are the fossils invaluable, therefore, to the Evolutionist without reference to their age, but the order in which they have appeared, and their relative age so far as it is possible to determine it, are in perfect harmony with the conclusions we have drawn from the structure of living plants and animals. Remembering the uncertainty attached to the absolute and relative age of rocks, let us examine the Primary, Secondary, and Tertiary Ages through which North America has probably successively passed, without reference to the relation these Ages bear in time to the corresponding parts of Europe, etc. Geologists subdivide the Primary, Secondary, and Tertiary Ages into periods (epochs) more or less characterized by their fossils.
AGE OF MOLLUSCA AND ALGAE.
Passing from the Azoic rocks, in the northern part of the State of New York, through the Potsdam region, to Trenton Falls, southwardly to the Helderberg Mountains near Albany, and eastwardly to Niagara, the immense number of fossil shells, particularly Brachiopods (Fig. 145), attracts the attention of the traveler. The Brachiopods of the present seas are few and far between, whereas the sea of that most ancient period was characterized by shells of this order; the remains of other Mollusca are found, but much less abundantly as compared with those of Brachiopods. The seas of this period must have swarmed with Crinoids, from the great number of them found petrified, their broken stems being known as Lily Stones (Fig. 146) and St. Cuthbert beads. The young of the Comatula, long supposed to be a distinct animal, the Pentacrinus (Fig. 42), is the only known representative of the Crinoids at the present time. With the Crinoids are also found abundantly
