428 
MR. ROBERT GURNEY ON THE CRUSTACEA 
On the other hand, a group of species may be distinguished 
which habitually live in a salinity less than that of the sea- 
water, but in which fresh- water species are very rarely found. 
These species are : — 
Ectinosoma curticorne, Bceclc. 
Dactylopusia tisboides (Claus). 
Stenhelia hispida, Brady. 
Canthocamptus palustris, Brady. 
Mesochra lilljeborgii, Bceck. 
Delavalia palustris, Brady. 
Sph.eroma rugicauda, Leach. 
„ serratum, Leach. 
Idotea viridis (Slabber). 
J.era marina (Fabricius). 
The species which I have only found as immigrants with 
exceptional salt tides are the following : — 
Centropages typicus, Kroyer. 
,, hamatus (Lillj.). 
Temora longicornis (Muller). 
Acartia discaudata, Giesb. 
,, longiremis (Lillj.). 
Euterpina acutifrons (Dana). 
Cyclopina littoralis (Brady). 
,, gracilis, Claus. 
Though the first of these groups include several species 
which are generally considered to belong to the second or 
even to the third, so that one may say that, in these rivers, 
marine types range into water of exceptionally low salinity 
and into conditions exceptionally unstable, yet it is surprising 
that the range of some species is not even greater than it 
proves to be. The existence in large European lakes of 
species of a marine type, such as Mysis relicta, of the many 
marine forms in fresh water in certain tropical countries, and 
of such genera as Cirolanides and Ciecosphaeroma in springs 
and deep wells shows that marine species are eminently 
capable of adaptation to fresh-water conditions. It would 
be thought that here, with the gradual transition from salt 
