MUSEUM OF COMPAEATIVE 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- 

 tal6s 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 ot 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 

 faun£e 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. S. 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 yarious names. 

 Prof. A. Agassiz has termed the archibenthal area tlie " continental region." 



