264 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
Sea is rather cooler as a whole than our Gulf in summer, its warmest 
water, off the coast of Belgium, being about 62.5°; with the greater 
part of its surface area 55°-60°. But nowhere in the North Sea are © 
the surface temperatures of summer as low as they are in the north- 
east corner of the Gulf of Maine. At fifty fathoms the temperatures 
of the North Sea and of the Gulf are about the same, though the 
range is somewhat greater in the latter, the extreme limits being from 
38°-48°. And they also agree closely in greater depths, which, in the 
North Sea, are limited to a small area at its northern entrance. 
Thus the 100 fathom temperature of the North Sea is between 41.9° 
and 44.6°; the temperature of the Gulf between 38° and 46° at that 
depth. 
' The surface density, at the temperature an situ, like the salinity, is 
considerably higher, as a whole, in the North Sea than in the Gulf of 
Maine. In summer the densities of the two overlap, that of the Gulf 
ranging, from about 1.0227 to about 1.0254; the North Sea from about 
1.0247 to 1.0266. But during the rest of the year the density prob- 
ably does not rise as high anywhere in the Gulf as in the North Sea. 
And in May the difference is great, for at that season, owing to the 
inrush of fresh river water, the surface density of the western side 
of the Gulf of Maine falls below 1.023, whereas in the North Sea it 
ranges from 1.0263-1.0273. Subsurface densities, likewise, are lower 
in the Gulf, for, while the temperature is not very different from that 
of the North Sea, the salinity is much lower. 
In short, there is nothing in the temperatures to cause any faunal 
difference between the two bodies of water, but the difference in 
salinity is so great that it might well have some influence. And it 
would not be surprising to find that the density was an important 
factor in determining the fish fauna of our Gulf by governing the 
flotation of pelagic eggs. 
THE COAST WATER AS A BIOLOGICAL ENVIRONMENT. 
The hydrographic facts outlined in the preceding pages have a 
twofold interest: first for their bearings on the general problems of 
oceanography; secondly for their relation to the animal population 
which the coast waters support. As a biological environment, the 
different parts of the continental shelf differ greatly, though all are 
characterized by relatively low temperature and salinity. The Gulf 
of Maine, except for its uppermost layers, is a region of great physical 
