500 Proceedings of the Royal Society of Edinburgh. [Sess. 
is often encountered. In the case of the genera Leptothrix and Gallionella 
the periods of growth are limited both in space and time. The bulk of 
the deposit of the iron-waters seems to have been made up of the remains 
of organisms, the growth of which, though limited as regards space and 
time, yet took place at frequent intervals. This admits of an easy physio- 
logical explanation, into which I need not at present enter. All my 
observations of Spirophyllum were made from material in which the 
bacteria were in a state of growth. Diagrammatic representations of the 
two organisms in question are given in figs. 1 and 2. 
I. The appearance of Gallionella is that of a thread spirally twisted 
Fig. 1. Fig. 2. 
round itself, whereas Spirophyllum is a band-shaped structure. The 
primary forms of the two organisms are thus fundamentally different. 
II. As will be seen by a perusal of my former paper,* my observations 
were taken from such favourable material that I was able to follow the 
whole of the life-history, and, by comparison of the individuals of various 
ages, to trace the method of growth. Small, spirally twisted bands could 
be seen that were no wider than the conidia from which they had 
germinated, so that it was evident that the peculiar spiral band shape had 
been assumed immediately on emerging from, or elongating from, the 
conidium. Again, the comparison of individuals in various stages of 
development showed that no departure was made from this spiral band 
form during the whole period of its existence. I regard this fact as of 
paramount importance in connection with this point, because it is evident 
* Ibid. 
