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CHAP. XIX.] FISHES. 467
covered belong to the Ganoidei, a highly developed group which
has continued to exist down to our times, and of which the
sturgeon is the best known example. We may therefore be sure
that the Upper Silurian rocks in which these are found, although
so very far back in geological history, do not by any means lead
us to the time when the primitive fish-type appeared upon the
earth. In the Carboniferous and Permian formations numerous
remains of fishes are found, allied to the Lepidosteus or Gar-pike
of North America. The next group in order of appearance, are
the Plagiostomata, containing the existing Sharks and Eays.
Traces of these are found in the highest Silurian beds, and become
plentiful in the Devonian and Carboniferous formations
and in all succeeding ages, being especially abundant in Creta-
ceous and Eocene strata. The Holocephali appear first in the
Oolitic period, and are represented by the living Chimaridse.
The Dipnoi, to which belong the Lepidosiren and Ceratodus, are
believed to have existed in the Triassic period, from the evidence
of teeth almost identical with those of the existing Australian
fish. All the ancient fossil fishes belong to the above-mentioned
groups, and many of them have little resemblance to existing
forms. The Teleostean fishes, which form the great bulk of
those now living, cannot be traced back further than the Creta-
ceous period, while by far the larger number first appear
in the Tertiary beds. The Salmonidse, Scopelidse, Percidse,
Clupeidae, Scombresocidse, Mugilidse, and Siluridse, or forms
closely allied to them, are found in the Cretaceous formation.
In the Eocene beds we first meet with Squammipennes, Cypri-
nidse, Pleuronectidee, Characinidse, Mureenidse, Gadidse, Pedi-
culati, Syngnathidae, and Hippocampidse.
Most of these fossils represent marine fishes, those of fresh-
water origin being rare, and of little importance as an aid in
determining the causes of the distribution of liAdng forms. To
understand this we must look to the various changes of the
land surface which have led to the existing distribution of all
the higher vertebrates, and to those special means of dispersal
which Mr. Darwin has shown to be possessed by all fresh-water
productions.