178 BULLETIN OF THE 
water on the coast of Florida by Pourtalés, and by the “ Blake” in the 
West Indies, the form is more depressed, the shell far more delicate, 
the colors pale pearly tints of lemon and pink. It seems as if differ- 
ences of temperature and nutriment, as between the north and the 
tropics, were indicated in very similar ways, both by the dwellers in 
the deep sea and those which inhabit the land. 
It might be thought that in the abysses, of whatever latitude, the 
conditions would be so similar that we should find the same animal 
presenting few, if any differences, from whatever part of the ocean it 
might come. This is to some extent true of the great oceanic deeps 
away from the continental shores and archipelagos. There the water 
is always cold, and a certain and not very profuse mollusk fauna has 
been found widely spread; having apparently migrated from the polar 
regions, and perhaps especially from the south polar regions, into the 
deeps of both hemispheres. It is very necessary, in considering the dis- 
tribution of the deep-sea mollusks, to bear in mind the different values 
which the expression “deep sea” has had, and which, if confounded, 
would give rise to serious errors. 
Formerly, when dredging with the usual appliances in small boats, 
one hundred fathoms was considered extremely deep, and specimens 
from even half that depth were considered as having come from deep 
water. This was proper enough when the collections were compared 
with those from the shore between tides, or even from the adjacent 
region below tide-marks, but which supported a growth of algz, either 
ordinary sea-weeds, or the solid calcareous kinds known as  corallines. 
But when naturalists began to investigate at much greater depths, the 
old terms lost their meaning. 
For present purposes deep-sea mollusks may be taken to include all 
those living at depths too great to allow algz of any sort to flourish, 
the limit depending somewhat on the locality. Those living only above 
that limit would form the littoral fauna, which, roughly speaking, may 
be said to extend from the shores to about one hundred fathoms in 
depth. With them in suitable places would be mixed many deep-water 
forms, which extend their range to shallow water without being charac- 
teristic of it. 
The remainder of the sea would naturally be divided rather by tem- 
perature than depth. But the temperature itself is somewhat dependent 
upon the depth, the influence of the great warm currents of the ocean 
rarely extending below seven or eight hundred fathoms, and this depth 
corresponds roughly to a temperature of about forty degrees Fahren- 
