220 REPORT ON ALBATROSS MOLLUSCA DALL. 



It is therefore evident that in speaking of dredging we must revise 

 our terms and define tliein so as to conform more nearly to the new con- 

 ditions under which such work is done. 



The waters immediately adjacent to the shores were long ago divided 

 by Forbes and other pioneers in marine exploration into zones or areas, 

 according to the conditions characterizing them ; as, for instance, tbe 

 Laminarian zone or region of brown kelp, tbe Coralline zone or region 

 of stony algse, etc. But for general purposes and to contrast the areas of 

 the whole sea one with another, according to their chief characteristics, 

 we may now divide the entire sea bottom into three regions. 



Tbe first is that to which light can penetrate, land therefore where 

 marine vegetation can exist. This is the Litoral Region, and in a gen- 

 eral way, modified by special conditions at particular places, it may be 

 regarded as extending from the actual shore out to the limit of 100 

 fathoms. Beyond this it is practically certain that the light reaching 

 the bottom is insufficient for the growth of sea-weeds. Outside of this 

 the borders of the continents slope gradually to the bottom of the 

 ocean, which is found usually at a depth of about 2,500 fathoms. 



On the upper parts of these continental slopes the conditions are often 

 very favorable for marine life. Currents of comparatively warm water, 

 like the Gulf Stream, sweep along, bringing fresh pure water and sup- 

 plies of food to the animals along their track. The division between 

 the abysses and the slopes is rather a matter of temperature than of 

 mere depth, but the temperature itself is somewhat dependent on the 

 depth. The influence of the great warm currents rarely extends below 

 700 or 800 fathoms, and this depth corresponds roughly to a tempera- 

 ture of about 40° Fahr. Below this it diminishes as the depth increases 

 at the rate of about one-tenth of a degree to 100 fathoms, until the freez- 

 ing point is reached, though there is no reason to suppose that the 

 abyssal water ever actually becomes congealed. 



To this cold, dark area of the ocean bottom has been applied the name 

 of the Benthal or Abyssal Region. 



To the region chiefly on the continental slopes, between the Litoral 

 and Abyssal regions, 1 gave, some years ago, the name of the Archi- 

 benthal Region. 



These divisions have been recognized by various writers and have 

 had several terms applied to them. Those I have mentioned seem to 

 me as characteristic as any, and, in some respects, more convenient than 

 any I have heard used. 



Let us now consider the conditions under which life exists iu the 

 Abyssal and Archibenthal regions. It may be premised that the dif- 

 ferences between them are largely of degree and not of kind and do not 

 require that the two regions should be considered separately. 



The chief characteristics reside in the composition of the sea water, 

 including its contained gases; in the dynamic status of the deeps, espe- 

 cially iu relation to temperature and pressure; iu the mechanical quali- 



