340 Proceedings of the Royal Physical Society, 
The progenitors of the present Invertebrata of the Forth 
were thus exclusively boreal and arctic in character, in which 
respect they differed materially from the contemporaneous 
fauna of the west of Scotland. The latter fauna included, 
in addition to the true northern forms that came in with 
the arctic conditions, many others whose constitution and 
previous history enabled them to adapt themselves to changed 
conditions, and to maintain their ground, more or less, along- 
side that held by the northern invaders. 
It was near the close of the Glacial Period proper, and 
when the land stood one hundred, or perhaps two hundred, 
feet lower than it does at present, and while the waters of the 
Forth were muddy with the clay liberated from the melting 
of the floating ice drifted both down and up its channel, that 
the boulder clays of Leith Docks, Portobello, Ele, Errol, etc., 
were formed. And it was at this period that the parent stock 
of the present molluscan fauna of the Forth first gained 
admittance here. This fauna has long been known to have 
possessed an exceptionally arctic character, which is strikingly 
shown by the nature of the shells found at Elie. 
No shells of any*kind have rewarded a very careful and 
oft-repeated search in the boulder clay at Leith Docks. It 
may well be, as Mr Peach has suggested, that at the time 
this clay was being formed the quantity of mud in the water 
was so great, and the surface temperature so low, that even 
arctic Mollusca could not well establish themselves. That 
they lived near here, however, is abundantly proved by the 
shells in the deposit of the same age at Elie. 
Under ordinary circumstances, and supposing that no 
important changes of level occurred, it might be expected 
that a colony of marine Mollusca established under these 
conditions would have lived on with but little modification, 
and with the disappearance of but few species, through the 
succeeding periods down to the present day. The changes 
of climate that have ensued since the Glacial Period embrace, 
it is true, some considerable oscillations of temperature; but 
then it has to be remembered that it is chiefly on the land, 
and to a very much lesser extent on the surface of the sea, 
that the influence of these climatal changes is made manifest. 
