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 Pays. 
Traces of these are found in the highest Silurian beds, and be- 
come 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 Chimseridsc. 
The Dipnoi, to which belong the Lepidosiren and (Jeratodus , 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 Salmonidae, Scopelidse, Percidee, 
Clupekke, Scombrcsocidee, 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, Pleuronectidse, Characinidee, Mursenidse, Gadidee, Pedi- 
culati, Syngnathidse, 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 living 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 fr^h-water 
productions. 
II h 2 
