s not inadequate representatives of this
order as it now exists. The Port Jackson shark, however,--a creature
that to the dorsal spines and shagreen-covered skin of the common
dog-fish adds a mouth terminal at the snout, not placed beneath, as in
most other sharks, and a palate covered with a dense pavement of
crushing teeth,--better illustrates the order as it first appeared in
creation than any of our British placoids.

[Illustration: Fig. 52.

PORT JACKSON SHARK.

(Cestracion Phillippi.)]

[Illustration:

Fig. 53.[13]
-----------------------
Silurian.
-----+-----------------Placoid.
| | Ganoid.
Old Red. | |
-----+----+------------
| |
Carboniferous. | |
-----+----+------------
Permian. | |
-----+----+------------
Triassic. | |
-----+----+------------
| |
Oolitic. | |
-----+----+----+----+--
Cretaceous. | | | | Ctenoid and Cycloid.
| | | |
-----+----+----+----+--
Tertiary. | | | |
| | | |
-----+----+----+----+--
Geologic [Pla. Gan. Cte. Cyc.] arrangement.
Agassiz's [Pla. Gan. Cte. Cyc.] arrangement.

THE GENEALOGY OF FISHES.]

[Illustration: Fig. 54.

AMBLYPTERUS MACROPTERTUS.

From the Coal at Saarbruck.

(A Ganoid of the Carboniferous System.)]

[Illustration: Fig. 55.

LEBIAS CEPHALOTES.

Cycloids of Aix. (_Miocene._)]

[Illustration: Fig. 56.

PLATAX ALTISSIMUS.

A Ctenoid of Monte Bolca. (_Eocene._)]

And here let me adduce another and very remarkable instance of the
correspondence which obtains between the sequence in which certain
classes of organisms were first ushered into being, and the order of
classification adopted, after many revisions, by the higher
naturalists. Cuvier, with not a few of the ichthyologists who preceded
him, arranged the fishes into two distinct series,--the Cartilaginous
and Osseous; and these last he mainly divided into the hard or
spiny-finned fishes, and the soft or joint-finned fishes. He placed the
sturgeon in his Cartilaginous series; while in his soft-finned order he
found a place for the Polypterus of the Nile and the Lepi