28 ANNALS OF SCOTTISH NATURAL HISTORY 
uncommon between tide-marks, and I have found it in rock pools 
above high water. 
In the study of these, as of many other organisms, it is found 
that when one tries to limit his observations to the so-called ‘‘ fresh- 
water” or to the “marine” species, the barrier set up between the 
two has to be more or less an arbitrary one, it being practically 
impossible to draw a line that will enable him to say “all on this 
side belong to the freshwater group and all on that to the marine.” 
We have in these brackish waters a kind of ‘‘no man’s land,” where 
the organisms of the sea and of the fresh water appear as if engaged 
in a perpetual struggle for the invasion of each other’s domain. 
Take, for example, the Veomyses already referred to. This Schizopod 
is found in the Firth of Forth, in the Moray Firth, and elsewhere in 
water that differs little from typical sea-water, and it belongs to a 
group of crustacea whose habitat is decidedly marine, yet this — 
species has been found in lochs such as Loch Wester in Caithness- 
shire, and Sinclair Loch in the Island of Barra in water which was 
quite fresh. On the other hand, we have Cyclops bicuspidatus—a 
typical fresh-water Copepod with 17-jointed antennules—represented 
in brackish-water pools by a form whose only apparent difference is 
that its antennules are 14-jointed, the difference being brought 
about by three joints having become coalescent with those next to 
them. ‘This variety (var. /wbbocki, Brady) is found associated with 
Cyclops bisetosus (another fresh-water Cyclops), Delavalia palustris, 
Canthocamptus palustris, Eurytemora velox, and others. Then 
again we have Cyclocypris serena, and Candona candida, so common 
in our fresh-water lochs and ponds, sharing the same pools with 
Cythere pellucida, Cythere gibbosa, and Cytheridea torosa, which are 
all more or less typical brackish-water species. It will thus be 
seen that this “‘borderland” presents a most interesting field for 
investigation. ; 
For the following species I am indebted to Mr. R. M. Clark, 
B.Sc., F.L.S., who obtained them in a shallow pool near Millden, 
about six or seven miles north of Aberdeen, and not far from the 
sea. The names of the species are as follows :— 
DIAPTOMUS CASTOR (/uvine).—Mr. Clark found this large and 
well-marked species moderately common in the pool referred to, and 
its occurrence there is all the more interesting from the fact that, so 
far as known to me, this is only the third time the species has with 
certainty been recorded from Scotland. In the “ Annals of Scottish 
Natural History” for July 1892, p. 202, I have a note on its occur- 
rence in the Braid Ponds near Edinburgh, but the place where these 
ponds existed has in recent years been greatly altered, and this 
Diaptomid is now probably extinct. The second was observed in a 
gathering of fresh-water Entomostraca collected in Helliers Water, 
Unst, Shetland, on 22nd June 1897, and sent to me by Mr. Robert 
