Page:Evolution of Life (Henry Cadwalader Chapman, 1873).djvu/81

Rh that they are but the links in a chain with the Fishes at one end and the true Reptiles at the other. Among the most perfectly preserved fossils are the Ichthyosauri (Fig. 58) and Plesiosauri. They seem, on the whole, to be more allied to Fishes in the structure of their paddles, backbone, etc., and Amphibia in other respects, than to true Reptiles, and must have diverged early from the main fish stem. Their position is somewhere near the Lepidosiren, Archegosaurus, and Labyrinthodon stems.

The Batrachia, or Amphibia, as they are often called, include the Frogs, Salamanders, Siredons, Tritons, Ccecilia, and the fossil Archegosaurus and extinct Labyrinthodons, They breathe by gills, at least at some period of their existence, and in this respect agree Mnth Fishes. Some of the Batrachia, as the Siren, Proteus (Fig. 61), and Menobranchus, retain their gills throughout life, and for this reason are called Perennibranchiata; whereas others, as the Frog (Fig. 64), lose them after passing through their tadpole stage. (Figs. 62, 63.) The Batrachia present two types for consideration: in the one we find the body covered over with bony plates or scales, as in the extinct Archegosaurus (Fig. 60), Labyrinthodons, and Coecilia; in the other, the body is naked, as in the Siren, Salamander, and Frog. A considerable advance in structure is seen on comparing the Batrachia with Fishes; but the Lepidosiren links together the Ganoid Fishes with the Frog division of the Batrachia, while the Archegosaurus leads up from the Ganoids through the Labyrinthodon to the Coecilia.

The Archegosaurus (Fig. 60), when first discovered, was supposed to be a fish; but more careful study has shown