WELLBURN : FISH FAUNA OF YORKSHIRE COAL MEASURES. 161 
gradually less and less salt, until no trace of its marine origin 
remained, and that the sharks and any other marine fauna present 
would adapt themselv^es (as they have been proved to do at the 
present da}^) to their gradually changing surroundings, and that 
the conditions now being favourable, fresh water fish would emi- 
grate from the surrounding rivers and streams, so that we should 
eventually have a fresh water lake with a fish fauna of both fresh 
water and marine types. 
In studying the life history of the Yorkshire Coal Fishes, 
that of Coelacanthus stands out as of special interest. These fish 
are present in great numbers in the Cannel Coal at Tingley, which 
coal appears to have been formed in a shallow lake of no great 
extent, the coal rapidly thinning out in all directions; and it seems 
highly probable that at certain seasons semi-stagnant and even 
dried-up conditions prevailed, and it was probably to meet these 
latter conditions that the fish C wlacanthus were provided with 
what may be termed their greatest " physiological peculiarity," 
their swim bladder, as it was probably by this means that they, 
like the lung fishes of the present day, were enabled to live 
during the dry seasons, their swim bladder acting — for the time 
being — as a lung. That the swim bladder did play such a part 
is rendered highly probable by their peculiar anatomical structure. 
Remarks on the Fish Remains. 
Sub-class : Elasmobranchii. 
Order : Ichthyotomi. 
Family : Pleuracanthidie. 
Genus: Fleuracanthus Agassiz, 1837. 
The Yorkshire coalfields have yielded the spines of several 
species of this genus, many being in a fine state of preservation, 
but others show all stages of erosion, some being destitute of 
denticles and blunt-pointed, and in these latter specimens, the 
superficial smooth layer having been removed, the spines appear 
of a fibrous texture. To the author C ompsacanthus triangularis 
Davis; C ompsacanthus major Davis; Phricacanthus hiserialis Davis; 
