BIOLOGY OF THE SCOTTISH LOCHS 
291 
by their specific names, are supposed to be peculiar to the abyssal 
region of lakes, or even of this particular lake. This number is 
given by Forel,^ but Dr Penard has since greatly extended the 
number of abyssal Rhizopods, whatever may have been done in 
other classes. 
In Scotland, abyssal life can only be spoken of with any confidence 
in the case of Loch Ness. The dredgings in Loch Morar, during the 
first year of the Survey, all failed, no doubt through lack of experience 
with the apparatus. Other lochs in which the dredge was used are 
not deep enough to be comparable with Loch Ness, though there was 
a similar fauna. ForePs definition of the abyssal region cannot be 
mechanically applied to Loch Ness. The central plain, though it 
exists in the principal lakes having the U-shaped cross-section which 
is supposed to indicate erosion by glaciers, is too limited in extent to 
be distinguished from the sloping sides, in biological studies at any rate. 
The steeply sloping sides of the Scottish lakes may account for an 
important difi^erence in the distribution of life in them. It has been 
found that a large proportion of the littoral species may in Loch 
Ness extend to a depth of about 300 feet, so that, if we gave the 
littoral and abyssal regions the same limits as in the Lake of Geneva, 
we would have a more extensive abyssal fauna than in that lake ; 
yet this fauna would have no peculiarities whatever entitling it to 
be called abyssal, but would be merely an extension of the littoral 
fauna. 
It is necessary to have a definition of abyssal which will fit the 
peculiar conditions of the Scottish lochs. The term is here used, 
then, to indicate those species of animals (and plants, if there were 
any) which are so well established in or on the muds of the 
bottoms of the lakes that they are generally distributed over the 
mud, and likely to be found in any part examined. Thus defined, 
a well-marked association of animals appears, the permanent in- 
habitants of these muds. They occupy the central plain, if you 
will, but none of them are peculiar to it. They all extend into 
the littoral region, from which I suppose them to be derived. The 
region is defined by one negative character, the absence of all other 
species. 
The depth at which the abyssal region begins varies in different 
lochs. In Loch Ness the abyssal association of species is fairly pure 
at depths of over 300 feet, a few littoral species being casual at 
greater depths. In Loch Earn the same association is found almost 
as pure at 200 feet. In St Mary's Loch it is found at 100 feet, but 
with a larger proportion of casual species. 
The small abyssal association comprises : — 
1 Le Leman, t. iii. p. 242, 1904. 
