42 
DAVIS : FOSSIL FISH EEMAINS. 
older Silurians. The fishes found in the Old Red are comprised in 
the externally armour-plated fishes of which Coccosteus may be 
taken as the type, and the thick, enamel-scaled fishes represented 
by Cheirolepis, Acanthodes and Osteolepis, all of which are 
ganoids. In the Mountain Limestone, Ganoid fishes are almost 
entirely absent, being represented only by a few plates of Coel- 
acanthiis from Armagh ; unless it shall be found that the various- 
shaped pustulate plates of Oracanthus represent the external 
armour of a fish allied to Coccosteus, 
The great Elasmobranch fishes, armed with dorsal fin-spines 
of great power, for either offence or defence, became of great 
importance during the limestone era, they are represented in 
earlier formations only by the small spines of Onchus in the Silur- 
ians. In later formations some of the Limestone Elasmobranchs 
are represented by descendants in the genus Ctenacanthus ; others 
may also possess characters more or less common to the genera of 
both formations, In the shales of the coal measures, ganoid fishes 
of very large size such as Rhizodus and Megalichth^^s become of 
frequent occurrence, the rare Coelacanthus of the fimestone, in the 
Cannel coal becomes an extremely abundant g-enus, and several 
placoid fishes possessing characteristics quite different from those 
of the Mountain Limestone becomes tolerably abundant. The Acan- 
thodians, which were common in the Old Red Sandstone strata, but 
absent from the limestone, again put in an appearance in the coal 
measures. The great majority of the limestone fishes, however, 
are not found in the coal measures, the great group of Copodonts 
and Psammodonts, the Cochliodonts and the last remnant of the 
Coccostean group, if Oracanthus be such, have all disappeared 
prior to the deposition of the coal measures. 
It must not of necessity be supposed that the changes indi- 
cated above were of univeral significance, it is far more probable 
that they were due to circumstances more or less localized in 
extent. The Carboniferous Limestone is a deep water formation, 
