222 CATALOGUE OF THE FOSSILS OF THE PERMIAN SYSTEM 
ceeds a yard in thickness, and sometimes is either entirely ab- 
sent or represented by a few thin yellowish bands, which 
alternate with the compact lime-stone. It is generally of a dark- 
grey colour, graduating into yellow or buff, in several localities. 
When first quarried it is very close and refractory, but after a 
short exposure to the weather, it easily splits into thin lamin 
or plates. This bed is exceedingly interesting on account of the 
numerous remains of fishes which are preserved in it. They are 
found, pressed quite flat between the lamine of which this bed 
is composed, in the following localities :—Whitley Quarries and 
Cullercoats Bay, Northumberland ; Boldon Hill, Houghton-le- 
Spring, Quarrington Hill, Thrislington Gap, East Thickley, Mid_ 
deridge, Aycliffe, and a few other localities in the county of Dur- 
ham. A few marine plants and a shell or two also occur in this 
bed. 
All the fishes of this formation belong to the Ganoid order, 
and have the vertebral column continued into the upper lobe of 
the tail. They are covered with stout, rhomboidal, highly enam- 
elled scales, which are strongly articulated together, and so 
placed that one row overlaps another in a manner similar to the 
arrangement of tiles or slates on the roof of a house. This may 
account for the external covering being oftentimes so perfectly 
preserved, though the internal structure has, in most cases, entirely 
disappeared. In a few instances, however, traces of a vertebral 
column and spinous processes extending to the fins are distinctly 
visible. Very few fishes belonging to the Ganoid order now exist, 
and these few are very rare, and confined almost to the inter-tro- 
pical rivers of Africa and America. This circumstance taken in 
connection with the fact that the older Palwonisci, and several 
other genera, are found in fresh-water limestones and shales, con- 
taining land plants, is a strong presumptive proof that these 
fishes also inhabited rivers, which, probably, at this period, flow- 
ed through the coal-measures. It has been conjectured from the 
contorted forms of many of the specimens that they were sud- 
denly destroyed by an infusion of sulphuret of copper into the 
waters they inhabited.* The great quantity of copper ore con- 
* Agassiz, Poiss. Foss. II., p. 70. 
