DAVIS : DISTRIBUTION OF FOSSIL FISHES. 
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. some particulars close relationship with the Elasmobranchs ; the 
bulbus arteriosus, with its many valves, more numerous in the 
ganoids, the chiasmic, non-decussating optic nerves and the spiral 
valve in the intestines, are all present, whilst in no other fishes are 
these characters to be found. There are other characters in which 
the ganiods differ from the sharks. In the latter the branchial 
arches are attached to the outer wall of the gill-cavity, the water 
passes by a number of slits in the dermal covering, usually five in 
number, and there is no operculum. The ganoids have free gills 
and an operculum, and in this respect they have near affinities 
with the Teleostei, whose gills are not attached to the walls of the 
gill-cavity, and they are protected by an operculum which forms 
the outer wall. There are other important characters in which the 
Ganoids resemble the Teleostei, notably in the possession by some 
of them of an air-bladder, which is preserved in the fossil state in 
the Cselacanths ; and also in the fact, that many of the Teleosteans 
are possessed of a dermal covering of ganoine, developed from a 
membranous skin. I do not intend to enter fully into the several 
reasons for supposing that some of the carboniferous fishes exhibit 
characters or affinities which render their near relationship with the 
Siluroid Teleosteans probable, but further discoveries of fossil forms 
more especially in beds which are of decidedly fresh- water origin, 
will most likely reveal closer alliances between the fossils and the 
recent Teleosteans, than has hitherto been considered probable. 
The knowledge of the mode of life and of the habits of the 
fish, to be gleaned from a careful consideration of their distribution 
and deposition, and the zoological affinities of their living allies, is 
perhaps not very extensive or decided, but there are a few facts 
which may be usefully remembered. The fish remains, in nearly 
every instance, are found on the surface of a bed of coal ; instances 
rarely occur in which the remains of fish or other vertebrates have 
been discovered in the shales above the coals ; in the sandstones, 
no instance has been recorded. There is thus, then, in every 
instance where they have been found a peculiar aggregation of 
fishes, whose modern representatives are of both marine and fresh- 
