INTRODUCTION. 11 
small size of the specimen, it would have been practically impossible to procure 
when the portion of the bed containing them was brought to the bank. It is also, 
unfortunately, only too true that miners are very fond of deceiving collectors, and 
handing them specimens which have been obtained from quite different localities ; 
and had I not known the rocks and localities well, I might often have been 
deceived by marine fossil forms (often Carboniferous) said to have been obtained 
from certain horizons, which, happily I knew, could not have been the case. 
A strong point in favour of the fresh-water origin of much of the coal-measures 
is brought forward by Prof. A. H. Green (‘ Coal and its Uses,’ edited by Thorpe, 
1878, p. 31). When discussing the want of unanimity among observers on this 
subject, and the distinction of the fauna of the Carboniferous Limestone from that 
of the Coal-measures, he says, ‘‘ They [the fauna] are rarely, if ever, found 
together. One, we know, was marine; is it not, then, to say the least, likely that 
the other was fresh-water?” And in support of this view he says, ‘‘ No marine 
amphibian is known, and we may therefore conclude that these [ Labyrinthodonta | 
were fluviatile in their habit.” 
In the same volume, p. 154, Prof. Miall, speaking of the same subject, says an 
examination of the fossils of the Coal-measures, apart from marine bands, gives a 
result strongly in favour of the fluviatile or terrestrial origin of the bulk of the 
Coal-measures ; and in adopting this conclusion we have no difficulty to face, 
except that implied by the supposition that the Elasmobranchs were more largely 
fluviatile than in other periods.”’ 
To sum up then, on the whole, very little positive evidence can be brought 
forward on behalf of a marine habitat for these shells; while there is, to say the 
least, a fair amount of presumption that they lived under fresh-water conditions. 
It may be, however, that we have in the occurrence of Carbonicola and Naiadites 
with various marine forms at the base only of the Coal-measures, an example of a 
genus changing its habitat. It is probable that all molluscan forms were originally 
marine in habit, and that by modification and adaptation some few forms became 
able to exist in fresh water; and it is also probable that this adaptation took 
place at different times, and is even going on now,—to wit, the discovery of 
Hydrobia Jenkinsi in inland canals of late years, which shell had till that time 
been only known under estuarine conditions. 
