MUSEUM OF COMPARATIVE ZOOLOGY. 179 
heit. Below this it diminishes to the freezing point at the rate of about 
one tenth of a degree to one hundred fathoms, forming the area which 
will here be called the abyssal or benthal region. The area between 
the abyssal and the littoral regions, chiefly on the slopes of the conti- 
nental platforms, may be called the archibenthal area.* In the abyssal 
areas the temperature at the bottom is known to be quite uniformly 
cold, the supply of food sinking from the surface cannot vary much in 
kind or quantity, and the distribution of life is comparatively sparse and 
uniform, as might be expected. 
But it is not in the abysses that the chiefest treasures of the dredger 
are to be found, nor the richest abundance of species and individuals. 
For these we must look to the archibenthal region skirting the conti- 
nental shores or islands, where strong currents bring abundant food and 
change of water, especially on relatively steep slopes which descend 
from the hundred-fathom line toward the deeps; there it is that the 
richest harvest comes up in the trawl. Such spots were found by Pour- 
talés near the Florida reefs ; by the ‘‘ Blake” near Cape San Antonio and 
off Grenada ; by the ‘‘ Challenger” near St. Thomas; and by the Fish 
Commission off Martha’s Vineyard. This increase is due to a variety of 
causes. In the first place it is certain that warm waters are more favor- 
able to a diversity of development and increase of individuals than cold 
ones. They are more stimulating to the organization both of the mol- 
lusk and of the creatures which form its food, and both multiply in con- 
cert. Secondly, the mollusk fauna of such regions, beside its population 
derived by migration from the abysses, is made up in great part of forms 
related to and connected with those which have developed along the 
shores, which are constantly being carried by tide and other agencies into 
deeper water than that in which they originated. There a certain pro- 
portion of them continue to flourish, probably become more or less modi- 
fied by change of food and environment, and so contribute to the variety 
and number of the fauna. It is not always, perhaps not often, that 
the species of the archibenthal region originally derived from the shores 
are to be found on the shores immediately adjacent to the spot where 
they are dredged. Often the littoral and adjacent archibenthal mollusk 
faune are entirely, or almost entirely, dissimilar. This is the case off 
the coast of Africa, or off the coast of New England, as observed by the 
naturalists of the U. 8. Fish Commission and the French expedition on 
the “Talisman.” But either in the far north or in the tropics we 
* These areas have been generally recognized and called by various names. 
Prof. A. Agassiz has termed the archibenthal area the “ continental region.” 
