Page:Evolution of Life (Henry Cadwalader Chapman, 1873).djvu/82

60 equal affinities with the Batrachia. The Labyrinthodon is another extinct form, with a very large skull, sometimes three feet in length and two in breadth. The bones of the skull in Archegosaurus and Labyrinthodon recall strongly the skull of the Gar-pike and Sturgeon. The persistence of a gristly backbone in Archegosaurus is the same as in the Sturgeon. The Lepidosiren and Archegosaurus agree in the structure of their backbone, and the retention of the branchial (gill) arches, and in the manner in which their skulls are joined to the backbone (absence of occipital condyles). The teeth are of the same kind (labyrinthic) in the Gar-pike, Archegosaurus, and Labyrinthodon. The large throat-plates in Archegosaurus are like those of Megalichthys (fish) and the Gar-pike; whereas, in the structure of the jaws (Fig. 60, B), certain bones called hyoid, and in the shoulder-girdle and extremities (Fig. 60, C, D), we see striking proofs of the relation of Archegosaurus to Batrachia like Proteus, whose jaws and extremities are (Fig. 60, b, c, d) very like those of Archegosaurus. The Archegosaurus form is the link between the Fishes and Batrachia, on the one hand leading through the Labyrinthodon to the Ccecilia, on the other to the Frogs through Siredon forms. The Archegosaurus came either directly from the Ganoids, or indirectly through the Lepidosiren. Supposing the latter view to be the true one, then the Ganoids divided into two branches, one being transformed into the common fish, the other giving rise to Lepidosiren-like forms, these leading insensibly to the Archegosaurus, the earliest of the Amphibia, the long type represented by Labyrinthodon and Coecilia forming one stem, the Siredon and Frogs, naked Amphibia, the other. The naked Batrachia are among the most striking proofs of the truth of the Derivation theory, as the links are all living. The Siredons and Proteus (Fig. 65, 61) strongly resemble the Lepidosiren and Archegosaurus; they have tails and external