V °1889. n '] PROCEEDINGS OF THE NATIONAL MUSEUM. 227 



Many species must Lave been eliminated to begin wit!). 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, 

 at least among the newer comers, 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 en- 

 vironment. We should also expect to find a certain proportion of archi- 

 benthal 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 m 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 hitter are positively ascertained. 



With the lesser need of protection from enemies and competitors 

 would necessarily be related a less vigorous elimination of characters 

 which in struggle and competition might prove sources of weakness. 

 The limits of uninjurious variation would be relaxed at the same time 

 and to the same extent We find, as we should expect, that the deep- 

 sea mollusks are more variable in their ornamentation and other super- 

 ficial characters than those from shallow water. In some species the 

 balance of characters is fairly well maintained, in others variation runs 

 riot, and it is impossible to say what amount of it should constitute a 

 basis for specific subdivisions among individuals. 



In general, deep-sea shells present pale or delicately tinted color-pat- 

 terns, are white or owe their color to the tinting of the epidermis. 

 This may be due directly to the absence of light. Sunlight, when 

 present, seems to have a stimulating effect in developing colors as is 

 shown by the greater brightness of tropical litoral shells whatever their 

 colors. It operates indirectly by promoting thV development of color 

 in algae which are fed upon by phytophagous mollusks and affect the 

 coloration of the latter directly through the assimilation of the coloring 

 matter of the food, mechanically. Indirectly, through the influence of 

 protective mimicry, the coloration of shells which frequent beds of se.i- 

 weed or rocks covered with stony algse is often modified in harmony 

 with the environment even when the species is not phytophagous. In 

 the deeps these influences are wanting, and the development of color 

 is necessarily the result either of uueradicated hereditary tendency, or 

 of some physical features of the environment which operate mechanic- 

 ally and are not yet understood. 



The colors chiefly affected by deep-sea mollusks are pink or reddish, 

 straw-color, and various shades of brown. These are found in the shell 

 and are more or less permanent. The epidermis of deep sea shells is 

 usually pale yellowish, but frequently is of a delicate apple-green such 



