336 THE FUESH-WATER LOCHS OF SCOTLAND 
The islands (Shetlands, Orkneys, Lewis, North Uist, Benbecula, 
Mull, and Lismore) with which I am dealing in this present paper 
all have the northern type of plankton, but they do not agree 
together so completely as might have been expected ; and the 
differences are sufficiently marked to enable one to readily distinguish 
between a tow-netting from the Shetlands, the Orkneys, or Lewis. 
In reality the Lewis plankton is very like, though not quite identical 
with, that of Ross-shire, whilst the Shetlands and Orkneys have each 
a more distinctive plankton. Nevertheless, if a complete list of all 
the species found in the Shetland lochs be compared with lists from 
the Orkneys oi* from Lewis, the differences will appear rather small, 
being for the most part varietal rather than specific. In this paper 
I am dealing with only three species of the plankton assembly, viz. 
Daphnia longispmn^ Bosmina ohtusirostiis^ and Ceratium Mrundinella^ 
and it will be seen that the Daphnia^ and to a less extent the Bosmina^ 
are present as fairly definite varieties in the several regions considered. 
I think it very probable that investigation would bring out very 
similar facts for a number of other species of the plankton, e.g. 
Bytliotrephes longimaniis^ Leptoclora hyaVina., various Rotifers and 
Desmids. 
It should be mentioned that the material available for the 
following comparisons was simply the tow-nettings taken on the 
various lochs at the time when the Lake Survey happened to be 
stationed there ; consequently each loch is represented by only one 
gathering ; but as the various islands were visited by different 
members of the Survey staff' at about the same time of the year, 
and that the best season for plankton-collecting, a comparison is 
justifiable. An examination of plankton from a number of lochs 
on each of these islands has provided the following distinctive 
characters : — 
For the Shetlands, examined 30th June to 11th August 1903, 
a very definite variety of Daphnia^ a small form of Bosmina ohtusi- 
rostris, a marked abundance of Diaptomus wierzejskii^ and apparently 
total absence of Diaptomus laciniatus^ a general rarity of Holopedium 
gihherum^ Bytliotrephes longimanus., Leptodora hyalina, Polyphemus 
pedimhcs, and Diaphanosoma hrachyiLra^ and a great abundance of 
Ceratium hirundinella. 
For the Orkneys, examined 15th to 27th August 1903, another 
variety of Daphnia ; otherwise much like the Shetlands. 
For Lewis, examined 10th July to 21st August 1903, Daphnia of 
a smaller type, a larger form of Bos7mna^ a marked abundance of 
Diaptoinus laciniatus and apparently absence of Diaptomus gracilis^ an 
abundance of Holopedium gihherum^ Leptodora hyalina^ Diaphanosoma 
hrachyura^ Bytliotrephes longimanus^ and Polyphemus pediculus, rarity 
