LEPIDOGANOIDEI. ] 59 



entombed in the primaeval mud have had their share. For, 

 just as a plaster-cast boiled in oil derives greater density and 

 durability from that addition, so the oily and other azotized 

 and ammoniacal principles of the decomposing fish operated 

 upon the immediately surrounding sand so as to make it 

 harder and more compact than the sediment not reached by 

 the animal principles. Accordingly it has happened that in 

 the course of the upheaval and disturbance of "old red" 

 strata, parts of it, broken up and exposed to the action of 

 torrents, have been reduced to detritus, and washed away, 

 with the exception of certain nodules, generally of a flattened 

 elliptic form, which are harder than the surrounding sand- 

 stone. Such nodules form the bed of many a mountain 

 stream in "old red sandstone" districts of Scotland. If one 

 of these nodules be cleft by a smart and well-applied stroke 

 of the hammer, the cause of its superior density will be seen 

 in a more or less perfect specimen of the fossilized remains 

 of some animal, most commonly a fish. 



But the placoganoid and lepidoganoid, heterocercal and 

 nOtochordal, fishes of the Devonian epoch existed in such vast 

 shoals in certain favourable inlets, that the whole mass of the 

 sedimentary deposits has been affected by the decomposing 

 remains of successive generations of those fishes. The De- 

 vonian flagstones of Caithness are an instance. They owe 

 their peculiar and valuable qualities of density, tenacity, 

 and durability, to the dead fishes that rotted in their primi- 

 tive constituent mud. From no other part of the world, per- 

 haps, can a large flagstone be got, which a builder could set 

 on its edge with assurance of its holding long together in that 

 position. A great proportion of the county of Caithness 

 formed, before its upheaval, the bottom of what may truly be 

 termed a " piscina mirabilis." Yet there are minds, who, cog- 

 nizant of the wonderful structures of the extinct Devonian 

 fishes — of the evidence of design and adaptation in their 



