［ A］The shellfish have a long history in the rock and many different kinds are known.
［ B］ Nevertheless, we know a great deal about many of them because their bones and shells have been preserved in the rocks as fossils. From them, we can tell their size and shape, how they walked, the kind of food they ate.
［ C］The first animals with true backbones were fishes, first known in the rocks of 375 million years ago. About 300 million years ago the amphibians, the animals able to live both on land and in water, appeared. They were giant, sometimes 8 feet long, and many of them lived in the swampy pools in which our coal seam, or layer, formed. The amphibians gave rise to the reptiles and for nearly 150 million years these were the principal forms of life on land, in the sea, and in the air.
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