DEVONIAN AND OLD RED PERIOD. 155 
Many of these “ fin-spines” have been preserved to us in the 
fossil condition, and the Devonian rocks have yielded examples 
belonging to many genera. As some of the true Sharks and 
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 Wacheracanthus, 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, 
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 
shallow water and littoral deposits of the period, while in the 
other we are introduced to the deep-sea accumulations of the 
same period. Nor can the problem at issue be solved by an 
appeal to the phenomena of the British area alone, be the 
testimony of these what it may. As a matter of fact, there is 
at present no sufficient ground for believing that there is any 
irreconcilable discordance between the succession of rocks 
and of life in Britain during the period which elapsed between 
the deposition of the Upper Ludlow and the formation of the 
Carboniferous Limestone, and the order of the same phe- 
nomena during the same period in other regions. Some of 
the Devonian types of life, as is the case with all great forma- 
tions, have descended unchanged from older types; others 
pass upwards unchanged to the succeeding period: but the 
fauna and flora of the Devonian period are, as a whole, quite 
distinct from those of the preceding Silurian or the succeeding 
Carboniferous ; and they correspond to an equally distinct 
rock-system, which in point of time holds an intermediate 
position between the two great groups just mentioned. As 
