TAGE OF vappo.e 
AS 
KK ..mn = 
ZOOLOG Y. 59 
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 Ichthyosau ri (Fig. 58) 
and Plesiosauri. They seem, on the whole, to be more allied 
em 
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. 
BATRACHIA, 
The Batrachia, or Amphibia, as they are often called, 
include the Frogs, Salamanders, Siredons, Tritons, Ceecilia, 
and the fossil Archegosaurus and extinct Labyrinthodons. 
They breathe by gills, at least at some period of their ex- 
istence, and in this respect agree with Fishes. Some of the 
Batrachia, as the Siren, Proteus (Fig. 61), and Menobran- 
chus, 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 Archego- 
saurus (Fig. 60), Labyrinthodons, and Ceecilia; 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 
* See page 61 for further proofs of this among the Amphibia. 
