DAVIS : FOSSIL FISH REMAINS. 
45 
optic nerves, have entirely disappeared, and it is only by com- 
parison of the parts preserved fossil with similar ones in existiag 
fishes that we are enabled to affirm tliat as in the recent fish, the 
teeth, &c. bear functional relationship with the soft parts of the 
fish, so in the fossil state similar teeth must have borne a 
corresponding- relationship with the parts of the fish which 
have disappeared. That the remains which are preserved 
should be mixed up in almost inextricable confusion is not 
remarkable when it is remembered that after the cartilaginous 
portions of the fish, which connect its several hard parts, have 
become decayed, the latter, each separated from the other, are 
liable to be washed hither and thither by every tide or current, 
and to become widely separated and intermixed with remains of 
other fishes so that it is an occurrence of extreme varity to find 
even the teeth of a fish in so happy juxtaposition that they can 
be identified as pertaining to the same individual. 
In attempting to trace the affinities and relationships of 
fossil fishes to recent forms the divergence between them is in 
some cases apparently slight, whilst in others, characters are 
developed which, unless they be regarded as connecting links 
between an older fauna and the present one, are inexplicable. So 
far as there is any evidence at present known, the Lower Silurian 
and all preceeding formations are devoid of fish remains ; from 
which it may be inferred, that the advent of fishes took place 
during the deposition of the Upper Silurian Strata. During the 
succeeding Devonian and Old Red Sandstone age, immense 
numbers of fishes swam in the seas. In size they were little 
inferior and in structure they were scarcely less highly organized 
than the fishes inhabiting the waters at the present day. Notwith- 
standing this, they present many peculiarities which have long 
since disappeared and been replaced in following- ages by others, 
again to flourish for a while and in their turn disappear. The 
fishes of the earlier formation whilst perfectly organized present 
a much simpler fauna than in succeeding ones, and whilst the 
