Ganoids agree with the Sharks in the structure of their heart and optic nerves, and Polypterus has the spiral intestinal valve of the Sharks, but their skulls have true bones, and they possess a gill-cover (opercular appendage). In this respect they agree with the Teliosts, or bony fish, of the present day. If we compare the tail of one of our common fish, a Cod, or Shad, or Perch (Fig. 57), with the tail of a Shark (Fig. 55) or a Sturgeon, we see that in the Perch the end of the tail divides into two equal parts, whereas in the Sturgeon (Fig. 56) the tail divides unequally. The unequally-ending or heterocercal tail is characteristic of these Ganoid fishes, and the equally-ending or homocercal tail is equally characteristic of our common fishes; but the tail of the embryo of one of our common fishes ending unequally is as heterocercal as the tail of the Sturgeon. The embryo fish is composed also of gristle, as regards its backbone and skull. Hence the transitory stage or embryo condition of our common fish represents the permanent stage of the Sharks and Sturgeons,—a striking proof of the truth of the view that the Bony Fish, or Teliosts, are the posterity of the Ganoids and Sharks. Fossil Ganoids, like the Ccelacanthes, Holoptychii, Coccolepis, and Amia of the present day, were probably the ancestors of fishes in which the air-bladder has a duct, as seen in the Carp, Herring, Salmon, while they were probably the progenitors of those fishes in which the duct is absent or rudimentary, as in the Perch, Cod, Sole. We turn now to a consideration of the remaining order of fishes, known as Dipnoi, and represented by the Lepidosiren (Fig. 59) of South America and the African rivers. During the rainy season in Africa, large tracts of land are overflowed by the rising of the rivers. With the retreating waters are carried most of the fish; but the Lepidosiren remains, and, burrowing in the mud (hence its name of mud-fish), constructs a hole, leaving only a small opening for the passage of air. Exuding
