them, of large size, with great heads
like those of crocodiles. Others bore more or less resemblance to
enlarged _Ophiomorpha_.

Every one knows that frogs begin their existence in the water as
tadpoles, which have the habits and mode of life of fishes. Thus, the
class _Batrachia_ naturally conducts us to the class _Pisces_, the class
of true fishes. This class contains a prodigious variety of forms, and
is far more rich in species than any other of the classes before
enumerated--even that of birds.

The fishes most familiar to us--such as the perch, carp, mackerel, cod,
herring, sole, turbot, salmon, pike, dory, and eel--all belong to one
great order called _Teleostei_, and which is made up of what are called
"bony" fishes, though there are some bony fishes which do not belong to
it. To the same order also belong the Muroena, the electric eel
(_Gymnotus_), the flying fishes (_Exocetus_ and _Dactyloptera_), the
sucking fish (_Remora_), the pipe-fish and sea-horse (_Hippocampus_),
the diodon, the ostracion, the file-fish (_Balistes_), the largest of
all fresh-water fishes (_Sudis gigas_ of South America), with a
multitude of other forms.

Certain more or less singular Teleosteans are classed together in a
subordinate group of "Siluroids" (of which fish the _Silurus_ is a
type), and which group includes, amongst others, the singular, cuirassed
fish Callichthys.

A group of fishes, which is now very small, but which at an earlier
period of the world's history was very large, includes within it all
those fishes which will be hereinafter occasionally spoken of as
"Ganoids," as they compose the order _Ganoidei_. Of all the forms of
this order, the sturgeon is that which is least unfamiliar to us. The
Ganoids are mostly fresh-water fishes and consist of the spoonbill-fish
(_Polyodon_), the bony-pike (_Lepidosteus_), the African _Polypterus_,
the mud fish (_Lepidosiren_), and the curious Australian fish
_Ceratodus_, which last is a singular instance of piscine survival.

Another order, _Elosmobranchii_, is made up of the sharks, together with
the skates (or rays) and the curious _Chimaera_. Amongst the skates may
be mentioned the celebrated torpedo or electric ray.

The three groups above enumerated contain almost all known fishes, but a
few other kinds, all of lowly organization, constitute two other groups
of very different structure.

One of these groups is called _Marsipo-branchii_, and contains the
lamprey, the _