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 Neomysis 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 ly-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. litbbockii, Brady) is found associated with 

 Cyclops bisetosus (another fresh-water Cyclops], Delavalia palustris, 

 Canthocamptus palustris, Eitrytemora velox, and others. Then 

 again we have Cydocypris serena, and Candona Candida, so common 

 in our fresh-water lochs and ponds, sharing the same pools with 

 Cythere pelhtrida, 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 (Jurine). 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 



