76 MR. F. BALFOUR BROWNE ON AQUATIC COLEOPTERA 
Hydrobius oblongus, Enochrus bicolor, and Dytiscus circumflexus, 
which though not necessarily confined to brackish water, are 
more commonly found in it than elsewhere. 
If this explanation of the relationship between the Muck 
Fleet and the Coast districts is correct, how is it that the 
Bure species are for the most part poorly represented on the 
coast ? Ilybius fuliginosus, generally more common in' 
running than in stagnant water, is the only species with its 
best record in the Bure district, which has its second best 
record in the Coast district, every other such species having 
a worse than third best record in the latter district. 
I believe that the explanation of this apparent anomaly 
is that shelter, presumably in the form of temperature, is the 
localising factor with a number of the species, and that those 
which can withstand the varying salinity of the Muck 
Fleet district cannot withstand the exposure of the Coast 
district. Such an explanation would account for the relation- 
ship between the Ormesby, Ant, and Bure districts, which 
are widely enough separated and yet in which 26 out of the 
46 species having their best record in one of these districts 
have their second best record in another of them, and in 
which only one, already referred to. has its second best record 
on the Coast. 
Further, such an explanation would account for the 
relationships of the Hickling district. As I have said, this 
district is fairly well sheltered from the east by belts of trees. 
The water is slightly saline, but the salinity is, I believe, not 
very variable. Out of the nineteen species with their best 
record in this district, four show their second best in the 
Ant district, three in the Bure district, and two in Ormesby 
district, apparently indicating that shelter is important to 
them, but that salinity does not affect them, while three show 
their second best record on the Coast, apparently indicating 
that shelter is less important than salinity. 
Thus it seems to me that the occurrence of the species in 
the various sub-districts suggests that a number of them are 
distributed according to opportunities for shelter, while some 
few seem to be more dependent upon the salinity of the water. 
