CHAPTER XII 



DISTKIBUTION OF MARINE MOLLUSCA DEEP-SEA MOLLUSCA AND 



THEIR CHARACTERISTICS 



Marine Mollusca may be divided roughly into Pelagic and non- 

 Pelagic genera. To the former division belong all Pteropoda and 

 Heteropoda, and a large number of Cephalopoda, together with 

 a very few specialised forms of Gasteropoda {lanthina, Litiopa, 

 Fhyllirrhoe, etc.). Pelagic Mollusca appear, as a rule, to live at 

 varying depths below the surface during the day, and to rise to 

 the top only at night. The majority inhabit warm or tropical 

 seas, though some are exceedingly abundant in the Arctic 

 regions ; Clione and Limacina have been noticed as far rioi'th 

 as 72V 



The vertical range of Pelagic Mollusca has received attention 

 from Dr. Murray of the Challenger, Professor Agassiz of the Blake 

 and Albatross, and others. Agassiz appears to have established 

 the fact that the surface fauna of the sea is limited to a com- 

 paratively narrow belt of depth, and that there is no inter- 

 mediate belt of animal life between creatures which live on or 

 near the bottom and the surface fauna. Pelagic forms sink, to 

 avoid disturbances of various kinds, to depths not much exceed- 

 ing 150 to 200 fathoms, except in closed seas like the Gulf of 

 California and the Mediterranean, where the bathymetrical range 

 appears to be much greater.- 



Non - Pelagic Mollusca are, from one point of view, con- 

 veniently classified according to the different zones of depth at 



^ The distribution of some Pteropoda luis been worked out by I\Iunth(>, Bih. 

 Svensk. Ak. Handl. XII. iv. 2, by Pelseneer " Challenger" Hep., Zool., xxiii., anil by 

 Boas, Spolia Atlantica. 



^ Bull. Mus. C. Z. Harv. xiv. p. 202 ; xxiii. p. 34 f. 



