BULLETIN 39, UNITED STATES NATIONAL MUSEUM. [16] 



like tlie Gulf Steam, sweep along, bringing fresh, pure water and sup- 

 plies of food to the animals along tlieir 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 iniiuence of the great warm currents rarely extending below 

 seven or eight hundred fathoms, and this depth corresponds roughly 

 to a temperature of about 40° F. Below this it diminishes as the depth 

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

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

 the abyssal water ever actually becomes congealed. 



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

 of the Benthal or Abyssal region, while that between the Literal and 

 Abyssal areas has been designated as the Archibenthal region. 



DEEP-SEA MOLLITSKS. 



While the average collector may not hope to explore the recesses of 

 the Abyssal and Archibenthal regions, a few observations on the rela- 

 tive conditions which obtain there will assist in the comprehension of 

 the general subject of the life of marine mollusks, and may therefore 

 be permitted. 



Deep-sea mollusks, of course, did not origiuate in the depths. They 

 are the descendants of those venturesome or unfortunate individuals who, 

 by circumstances carried beyond their depth, managed to adapt them- 

 selves to their new surroundings, survive, and propagate. Many spe- 

 cies must have been eliminated to begin with. Others more plastic, or 

 more numerous in individuals, survived the shock and have gradually 

 spread over great areas of the oceanic floor. In accordance with these 

 not unreasonable assumptions, we should expect to find among the newer 

 comers at least some characters which were assumed under the stress 

 of the struggle for existence in the shallows, and which, through specific 

 inertia, have not become wholly obsolete in the new environment. We 

 should also expect to find a certain proportion of Archibenthal species 

 in any given area, identical with or closely related to the analogous 

 Litoral region forms of the adjacent shores. 



In the Abyssal region alone should we expect to find that any con- 

 siderable proportion of the fauna has lost all its litoral characteristics, 

 assumed characters in keeping with its environment and become dis- 

 seminated over the ocean bottom throughout a large part of its extent. 

 These expectations in the main are fairly satisfied by the facts as far as 

 the latter are positively ascertained. 



In order that their existence may be maintained the abyssal mollusks 

 require oxygen to aerate their circulation, food to eat, and a foothold 

 upon which they may establish themselves. It is necessary that the 

 conditions should be such as will not prevent the development of the eggs 

 by which successive generations are propagated, and that they do per- 

 mit it may be assumed from the very fact that mollusks in large num- 



