Page:Popular Science Monthly Volume 3.djvu/451

Rh surprise, for it is commonly supposed that these animals never leave the water for the land. It has been ascertained by naturalists that certain varieties of tropical fish often leave the water, and walk, or rather hobble, on the shore, using their fins for legs. This they can do for days together. They have also been known to climb trees in search of sustenance. Their track-way would be peculiar, consisting of two rows of dots or round impressions, made by the prominent sharp spine of the fin, accompanied by various trails produced by the shorter rays, body, and tail. One species of the Ichnozoa has made a trail so much like the markings of these tropical fish that we must believe the Siluridœ had other representatives in the Triassic waters. Another impression seems to have been made by an ordinary fish striking his fins against the ridges between ripple-marks in very shallow water.

With considerable hesitation, thirty-four species of Ichnozoa may be referred to insects. These animals generally have six feet. If attention be paid to the manner in which the common fly walks, it will appear that every foot is brought to the ground, each in a different place, so that every extremity makes a mark. A fly that has been immersed in a colored fluid, and then travels over a sheet of white paper, will leave impressions as distinct as those found upon stone, and the colored track-ways must be our guides to the affinities of the ancient ichnites, like the Copeza.



Between these ancient and modern impressions it is difficult to find marked differences, except of size. Each displays two rows of impressions in groups of three, the several clusters alternating with one another. Of the three marks or lines in each cluster, the inner is almost at right angles with the line of march, the central and outer point backward, the latter the most.

We may suppose that each group of markings was made by the three feet on each side of the animal, the inner impressions by the