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Proceedings of the Koyal Society of Edinburgh. [Sess. 
This affinity for iron is characteristic of a class of bacteria which are known 
as iron-bacteria, and is also found among certain algae, and among certain 
protozoa. So far this power is not known to be possessed by fungi. That 
the fossil mould under consideration possessed it during its lifetime the 
following facts render probable : — 
1. The deposition of iron on its membranes was not uniform in 
character. Some hyphae were thinly covered, thus showing the membrane 
as a sharply contoured line. Others had the membrane thickened two or 
three fold on account of the deposition, with the result that the membrane 
was made up of a broken irregular line of minute fragments of ferric 
hydroxide. Between these two extremes all intermediate stages were found. 
Now, if the threads of the modern iron-bacterium Leptothrix ochracea be 
examined, the same differences will be noted in the threads which make up 
the bulk of the deposits at the bottom of the ferruginous waters. In their 
case the nature of the iron deposit is an indicator of the time that has 
elapsed since the death of the thread in question. The simplest explana- 
tion of the similarity in the two cases is that in the fossil, as in the modern 
organism, the iron was laid down under the same conditions — that is, during 
the lifetime of the fossil organism. Had the iron entered subsequent to the 
formation of the rock, it is reasonable to conclude that on all the threads 
the iron deposition would have been of the same nature, and in so far as the 
deposition is concerned one thread would not have differed from another. 
2. It was noteworthy that the fragment of organic remains that had 
been penetrated by this mould was devoid of iron throughout its interior 
except in the spaces occupied by the threads of this mould. The organic 
fragments containing the mould thus differed essentially from the round or 
oval oolites which companioned them in the calcite matrix, and which were 
coloured uniformly reddish yellow throughout their whole substance. In 
many cases there was also a fringe of the reddish-yellow peroxide of iron 
round the outside of the organic fragments, but their interior was always 
quite clear, and it was this fact which made their observation a com- 
paratively easy matter. 
It is difficult to understand how the threads could have thus become 
selectively coloured by the iron compound if the iron in the stone had been 
wholly deposited by replacement at a period subsequent to the formation 
of the rock. So far as these hyphse are concerned, a simpler explanation 
of their presence is that they formed part of an organism that lived and 
died on particles of dead animals which littered the bottom of shallow 
ferruginous waters, and that the iron was collected during and immediately 
after their lifetime. 
