Many of these fishes, reptiles, and insects are unknown at the present day. They performed their work, if work they had to do, and disappeared. Their remains are found in the coal seams and in the rocks which lie above or beneath the coal, and form an interesting subject of study. Some of the reptiles were enormously large. Remains have been found of a lizard more than one hundred feet long, with an open countenance, that could have taken in an ordinary man about as easily as a chicken swallows a fly. The skeletons of these reptiles are found, and I think that most people who examine these skeletons are inclined to give a sigh of relief when they remember that such creatures are now extinct. They would be very disagreeable travelling companions, and one might be very much disinclined to meet them in a narrow lane on a dark night. Some years ago I examined the skeleton of a reptile discovered in the Mississippi Valley, and though the bones were cold and motionless, I had the wish to keep at a respectful distance from them. He had a mouth that reminded one of the extension top of a patent carriage; and when his jaw was pushed back, it seemed to me that he could have walked down his own throat without the slightest difficulty. [Sidenote: CONVERSION OF PEAT TO COAL.] The most plausible and reasonable theory of the formation of coal seems to be that it is for the most part the remains of vegetable matter which had become decomposed and changed to mineral on the spot where it remained and is now found. The fibrous tissues of the aquatic vegetation flourished like a thick carpet on the moist surface. It became mingled and matted together, as we now find turf and peat in peat bogs, and in swamps and marshes. On the borders of great lakes, which in time were built up and became swamps, these plains extended, and underwent slow depression. Layers of sand and other substances were carried down below the level of the sea, which we now find among and alternating with the coal seams in the shape of beds of shales and sandstones. Then another system of lagoons formed above them, and allowed new jungles to spring up and new marshes to be formed. These were in turn depressed and covered by the waters. In this way, step by step, the coal beds were built up. According to geologists, each coal seam represents a depressed swamp, while the intervening strata of sandstone, and shale, and clay, mark the various sediments which were brought together by the action of the waters.