ZOOLOGY. 57 
Ganoids agree with the Sharks in the structure of their 
heart and optic nerves, and Polypterus has the spiral intes- 
tinal 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 homocer- 
cal 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 em- 
bryo condition of our common fish represents the perma- 
nent 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 Coelacanthes, 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, 
2 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 repre- 
sented 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, leav- 
ing only a small opening for the passage of air. Exuding 
