LEPIDOGANOIDKI 1 35 



six of these short and small lateral branches on each side of 

 the medullary rays. The teeth of Dendrodus occur in the 

 corn-stone beds of the " Old Eed " at Scat-crag, near Elgin. 



Such are some of the forms and structures of fishes that 

 swam in the seas from which were deposited the sediment 

 that has hardened into the " old red sandstones" of Great 

 Britain, Russia, and other parts of the world. And in this 

 process of consolidation the carcases of the fishes 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 oi]y and other azotized and ammoniacal 

 principles of the decomposing fish operated upon the imme- 

 diately surrounding sand so as to make it harder and more 

 compact than the sediment not reached by the animal princi- 

 ples. 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 sandstone. 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 speci- 

 men 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 wholly to the dead fishes that rotted in their primitive 



