[Illustration: Fig. 4.--Cephalaspis and Loricaria, an Ancient and a Modern Armored Fish.] Pterichthys, the wing fish, was another small, quaint, armor-clad creature, whose fossilized remains were taken for those of a crab, and once described as belonging to a beetle. Certainly the buckler of this fish, which is the part most often preserved, with its jointed, bony arms, looks to the untrained eye far more like some strange crustacean than a fish, and even naturalists have pictured the animal as crawling over the bare sands by means of those same arms. These fishes and their allies were once the dominant type of life, and must have abounded in favored localities, for in places are great deposits of their protective shields jumbled together in a confused mass, and, save that they have hardened into stone, lying just as they were washed up on the ancient beach ages ago. How abundant they were may be gathered from the fact that it is believed their bodies helped consolidate portions of the strata of the English Old Red Sandstone. Says Mr. Hutchinson, speaking of the Caithness Flagstones, "They owe their peculiar tenacity and durability to the dead fishes that rotted in their midst while yet they were only soft mud. For just as a plaster cast boiled in oil becomes thereby denser and more durable, so the oily and other matter coming from decomposing fish operated on the surrounding sand or mud so as to make it more compact." It may not be easy to explain how it came to pass that fishes dwelling in salt water, as these undoubtedly did, were thus deposited in great numbers, but we may now and then see how deposits of fresh-water fishes may have been formed. When rivers flowing through a stretch of level country are swollen during the spring floods, they overflow their banks, often carrying along large numbers of fishes. As the water subsides these may be caught in shallow pools that soon dry up, leaving the fishes to perish, and every year the Illinois game association rescues from the "back waters" quantities of bass that would otherwise be lost. Mr. F. S. Webster has recorded an instance that came under his observation in Texas, where thousands of gar pikes, trapped in a lake formed by an overflow of the Rio Grande, had been, by the drying up of this lake, penned into a pool about seventy-five feet long by twenty-five feet wide. The fish were literally packed together like sardines, layer upon layer, and a shot fired into the pool would set the entire mass in motion, the larger gars as they dashed about casting the smaller fry into the air, a score at a time. Mr. Webster estimates that there must have been not less than 700 or 800 fish in the pool, from a foot and a half up to seven feet in length, every one of which perished a little later. In addition to the fish in the pond, hundreds of those that had died previously lay about in every direction, and one can readily imagine what a fish-bed this would have made had the occurrence