DEVONIAN AND OLD RED PERIOD. 



157 



Dog-fishes, some of the Ganoids, and even some Bony fishes, 

 possess similar defences, it is often a matter of some uncer- 

 tainty to what group a given spine is to be referred. One of 

 these spines, belonging to the genus Machceracanthus, from the 

 Devonian rocks of America, has been figured in a previous 

 illustration (fig. 102, /). 



In conclusion, a very few words may be said as to the 

 validity of the Devonian series as an independent system of 

 rocks, preserving in its successive strata the record of an 

 independent system of life. Some high authorities have been 

 inclined to the view that the Devonian formation has in nature 

 no actual existence, but that it is made up partly of beds 

 which should be referred to the summit of the Upper Silurian, 



Fig. 105. A, Polypterm, a recent Ganoid fish ; B, Osteolepis, a Devonian Ganoid 

 a, a, Pectoral fins, showing the fin-rays arranged round a central lobe. 



and partly of beds which properly belong to the base of the 

 Carboniferous. This view seems to have been arrived at in 

 consequence of a too exclusive study of the Devonian series 

 of the British Isles, where the physical succession is not wholly 

 clear, and where there is a striking discrepancy between the 

 organic remains of those two members of the series which are 

 known as the " Old Red Sandstone " and the " Devonian " 

 rocks proper. This discrepancy, however, is not complete; 

 and, as we have seen, can be readily explained on the sup- 

 position that the one group of rocks presents us with the 



