10 BIOLOGICAL SOCIETY OF WASHINGTON. 



viscid layer of the finest possible calcareous mud or clay. The 

 latter formation is meagre in its fauna as clay is when it occurs 

 in shallow water. 



Certain forms of mollusk-life flourish in a soft bottom espe- 

 cially the Nuculidcz and their allies which are notabfy 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 Echi- 

 noderm, something they must have for themselves 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 when we find Lcpctclla safely 

 housed in the tubes of dead Annelids or Hydroids, and Chor- 

 istes taking refuge in the empty ovicapsules of rays or sharks. 

 Small hermit crabs take to the tooth-shells (JDentalium) or to 

 the tubular Pteropods {Cuvierina); or Amalthea roosts on an 

 Echinus spine and builds for itself a platform as it grows, re- 

 calling 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 considerable magnitude. 



The action of erosion and solution for some reason seems less 

 potent here than in 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 this. 

 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 atten- 

 tion has hitherto not been directed, is worthy of mention. This 

 is the habit of certain fishes, which exist in vast numbers, of 

 frequenting certain areas where they eject the broken shells of 



