The land operculates give no help, and, of the carnivorous genera, _Rhytida_ is a marked link between Africa and Australia, while _Streptaxis_ is equally so between S. America and Africa. As regards fresh-water Gasteropoda, _Ampullaria_ is common to S. America and Africa, while _Isidora_ is common to Africa, Australia, and New Zealand, but is altogether absent from S. America. _Gundlachia_ occurs in Florida, Trinidad, and Tasmania, but has not been detected in Africa. It must be concluded, therefore, that the present state of the evidence which the Mollusca can afford, while exhibiting certain curious points of relationship between the three regions in question, is insufficient to warrant any decided conclusion. [Illustration: FIG. 237.--_Macrocyclis laxata_ Fér., Chili.] CHAPTER XII DISTRIBUTION 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 (_Ianthina_, _Litiopa_, _Phyllirrhoe_, 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 north as 72°.[380] 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 comparatively narrow belt of depth, and that there is no intermediate 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 exceeding 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.[381] Non-Pelagic Mollusca are, from one point of view, conveniently classified according to the different _zones of depth_ at which they occur. Thus we are enabled to distinguish Mollusca of (_a_) the _littoral_, (_b_) the _laminarian_, (_c_) the _nullipore_, or _coralline_, and (_d_) the _abyssal_ zones. It must be borne in mind, however, that these zones cannot be exactly defined, and that while the littoral zone may be understood to imply the area between tide-marks, and the abyssal zone a depth of 500 fathoms and upwards, the limits between the laminarian and the coralline, and between the coralline and abyssal zones can only be fixed approximately.