224 REPORT ON ALBATROSS MOLLUSCA DALL. 



subdivision, from something which may be described as calcareous 

 gravel to an impalpable mud which may or may not be dotted with 

 concretions of manganese, iron, or other mineral matter. The gravels 

 are chiefly confined to the Archibenthal Region; the true deeps are 

 generally carpeted with a viscid layer of the finest possible calcareous 

 mud or clay. The latter formation is meager in its fauna as clay is wheu 

 it occurs in shallow water. 



Certain forms of mollnsk life flourish in a soft bottom, especially the 

 Nuculidce and their allies, which are notably abundant in the depths as 

 well as in the muddy shallows of the Litoral Region. Others require 

 some solid substance upon which to perch, a stone, a bit of wood, a 

 spine from some dead echinodcrm, something they must have for them- 

 selves and for their eggs which shall raise them above the muddy floor. 

 In regions where such objects are rare or absent on the sea bottom such 

 mollusks are equally rare or wanting. Most ingenious are the shifts 

 made in many cases, as wheu we find Lepetella safely housed in the 

 tubes of dead annelids or Hydroids, and Ghoristes taking refuge in the 

 empty ovicapsules of rays or sharks. Small hermit crabs take to the 

 tooth shells (Dentalium) or to the tubular Pteropods (Cumerina), or 

 Amalthea roosts on an Echinus spine and builds for itself a platform as 

 it grows, recalling the arboreal houses of some Oriental savages. 



In the Archibenthal Region there is a more or less constant drift of 

 debris from the adjacent shallows which gradually forms banks of con- 

 siderable magnitude. The action of erosion and solution for some rea- 

 son seems less potent here thau iu either the shallower or the deeper 

 parts of the sea. In the shallower parts the excess of motion, in the 

 deeps the excess of the eroding agent, may account for tbis. The fact 

 is known to me from the study of many specimens from both regions 

 and is beyond question. 



A feature in forming certain of these banks, to which attention has 

 hitherto not been directed, is worthy of mention. This is the habit of 

 certain fishes, which exist iu vast numbers, ot frequenting certain areas 

 where they eject the broken shells of mollusks, corals, barnacles, and 

 other creatures which they have cracked, swallowed, and cleansed of 

 their soft tissues by digestion We have learned from Darwin of the 

 marvelous work of the earth worm in Britain. The ejectainenta of a sin- 

 gle fish of moderate size in one day would far exceed the accumulations 

 of many earth-worms for a much longer time. Now, in examining 

 critically large quantities dredged from the bottom, I have found the 

 material from certain areas almost entirely composed of these ejecta- 

 menta. In the interstices some small creatures hide, but the tooth 

 marks of the fish were upon nearly every fragment. As, for a pint of 

 fragments of a given species, this bottom stuff would rarely contain 

 half a dozen specimens which had been taken alive by the dredge (most 

 frequently the species did not occur at all living in the material so 

 dredged), it was obviously impossible that the shells could have beeu 



