BULLETIN 39, UNITED STATES NATIONAL MUSEUM. [20] 



For the naturalist of to-day the most interesting feature of abyssal 

 life is not that it furnishes him with singular and archaic forms, useful 

 in his study of extinct genera; nor the beauty and rarity of the creatures 

 living under such unusual conditions. The most important character- 

 istic of abyssal life is, that it, and it alone, exhibits a fauna in which 

 reciprocal struggle is nearly eliminated from the factors inducing var- 

 iation and modification. 



Hence the course of evolution and modification, though still com- 

 plex, is certainly much less so than in the shallower parts of the ocean. 

 For this reason we may hope to penetrate more deeply into its myster- 

 ies with deep-sea animals than with those less fortunately situated. In 

 this opportunity lies the chief importance of research into the biology 

 of deep-sea mollusks. Nowhere else may we hope to find the action 

 and reaction of the contending forces less obscure, and modification in 

 most cases has not extended so far that we can not compare the deei^- 

 sea forms with their shallow-water analogues and draw valuable con- 

 clusions. 



For an account of the methods and apparatus employed in deep-sea 

 researches the student may be referred to the following works : 



Three cruises of the U. S. Coast and Geodetic Survey Steamer Blake 

 in the Gulf of Mexico, etc., by Alexander Agassiz. Boston : Houghton, 

 Mifflin & Co., 1888; 2 vols. 8°. With many maps and illustrations. 



Deep Sea Sounding and Dredging (etc.), by Chas. D. Sigsbee, Lieut. 

 Com. U. S. Navy. Washington: Coast and Geodetic Survey, 1880. 4°. 

 With many illustrations and supplement dated 1882. 



MOLLUSKS OF THE LITORAL REGION. 



The litoral region may be divided into several subordinate areas, the 

 first of which is the beach, or litoral proper, between the extreme 

 range of the tide or high and low water mark. The next has been 

 called the area of sea- weeds or laminarian zone from the Laminaria, or 

 long-leaved kelp, which grows in it. Its extent varies in different parts 

 of the world, but in general is regarded as between the low- water mark 

 of spring tides and a depth of about 15 fathoms or 90 feet of water. 



Outside of the laminarian zone and extending to the limits of the 

 penetration of light into the depths is the coralline zone, where the 

 vegetation consists chiefly of stony algse, or nullipores, and in which 

 Polyzoa or corallines are most abundant. The outer limit of this zone 

 is usually taken as 100 fathoms (600 feet), an arbitrary limit, but approx- 

 imating to the truth. 



Most collectors find their chief resource in the beach, where they 

 obtain not only the mollusks proper to that region but many others 

 cast up by the sea which normally inhabit the outer zones. 



Those fortunate enough to secure the use of a rowboat and dredge^ 

 find a rich field accessible to them in the laminarian zone and the inner 



