4 CARBONICOLA, ANTHRACOMYA, AND NATADITES. 
which they lived ; or the periodical inundations of the sea might distribute marine 
forms in such a manner that they would be deposited in the same beds with fresh- 
water forms; or a river cutting its way through slightly older marine beds might re- 
deposit fossil forms with those living in its waters,—indeed, stratigraphical evidence 
as to condition is so liable to be misinterpreted, and so many different factors may 
have been introduced, causing a mélange of marine and fresh-water fauna, that 
more stress is to be laid on biological evidence, which, perhaps not itself altogether 
satisfactory when only a single genus is taken into account, becomes more satis- 
factory as other genera are brought forward as evidence of the main or predomi- 
nant conditions existing during the deposition of any bed containing them. When, 
in addition to strong presumptive evidence, the great bulk of stratigraphical obser- 
vations in the majority of cases would seem to show that the beds in which the 
genera Carbonicola (Anthracosia), Anthracomya, and Naiadites (Anthracoptera) 
occur are not of marine origin, from the all but universal absence of typical 
marine forms, it would appear to be safe to affirm a fresh-water or estuarine habitat 
for them. 
With regard to the views of Scotch geologists on this point, I quote from 
Dr. John Young’s memoir ‘On the Carboniferous Fossils of the West of 
Scotland” (‘ Trans. Geol. Soc. Glasgow,’ vol. iii, 1871, p. 44), where he says, 
speaking of the so-called Coal Unios, ‘‘ None of the shells or other fossils of the 
marine limestone series are found in the same beds with this group of molluses ; 
and it has occurred to me that in the one or two instances in which a single 
example of Anthvacosia is said to have been found associated with marine shells 
it may have been drifted from its proper habitat, or washed out of some 
older bed of fresh-water strata.” And again, in 1880 (‘ Trans. Geol. Soe. 
Glasgow,’ vol. vi, p. 228), “ As far as Scotland is concerned, I have never observed 
any commingling of true marine fossils in any of our mussel-band beds. The 
reptiles, fishes, molluscs, annelids, ostracods, and other organisms belong to 
genera and species that are seldom or never met with in marine hmestone strata. 
This being the case, I believe the opinion formerly entertained, as to the marine 
origin of the strata of our Upper and Middle Coal-measures, especially of the 
bed characterised by Anthracosia, was an opinion based on faulty observation, and 
which will yet be proved to be untrue when the fossils occupying each horizon of 
strata come to be critically examined. Oscillations of the earth’s crust may bring 
strata, which have been deposited under either lacustrine or estuarine conditions, 
into very close contact with those of true oceanic deposits, and this may be 
repeated again and again in the same series of beds. It becomes, therefore, the 
work of the paleontologist to say which of the organisms found in this com- 
mingling of beds properly belong to the sea, and which to lakes and rivers.” 
This is most valuable evidence, as beds of coal and ironstone occur in the 
