60 EVOLUTION OF LIFE. 
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 mánner 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 struc- 
ture 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, 2, 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 Labyrin- 
thodon 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 Cecilia 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 
Lepidosirenand Archegosaurus; they have tails and external 
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