XII WIDE DISTRIBUTION OF DEEP-WATER FORMS 363 



miles south-west of Australia, and also in 2650 fatli. off the mouth 

 of the Rio de la Plata ; Semele profundorum in 1125 fath. near 

 the Canaries, and in 2900 fath. mid N. Pacific; Verticordia 

 desliayesiana in 155 fath. near Cape York, and in 350 fath. off 

 Pernambuco ; Area pteroessa in 2050 fath. mid N. Pacific, in 

 1000-1675 fath. west of the Azores, and in 390 fath. off the 

 West Indies; Area corpulenta in 1400 fath. off N.E. Australia, 

 in 2425 fath. mid-Pacific, and in 1375 fath. near Juan Fer- 

 nandez ; Lima goliath in 775 fath. off S. Japan, and in 245 

 fath. off S. Patagonia; Pleurotoma engonia in 700 fath. north- 

 east of New Zealand, and in 345 fath. off Inoshima, A surpris- 

 ing range was occasionally found even in shallow- water species; 

 thus Petricola lajncida was discovered by the same expedition 

 in the West Indies and N. Australia, Cardita calyeulata off 

 Teneriffe and in Bass Strait, Area imbrieata off Cape York 

 and in the West Indies, 3Iodiolaria cuneata at Port Jackson 

 and Cape of Good Hope, Lima squamosa at Teneriffe and the 

 Philippines. In these latter cases it is not improbable that the 

 species lives in deep water as well, from which it has not yet 

 been dredged. 



It follows from these considerations that any attempt to 

 classify marine Mollusca under Regions and Provinces can only 

 apply to Mollusca which occur at moderate depths. The most 

 important factor in the environment, as determining distribu- 

 tion, is the temperature of the water, which is probably to be 

 regarded as affecting not so much the adult Mollusca as their 

 ova ; for the adult might possibly support life under conditions 

 in which the ova would perish. It appears that a sudden change 

 of temperature is the most effective barrier to distribution,^ and 

 may bring the range of a species to an almost instantaneous 

 stop, while a very gradual change will allow it to extend its 

 range very widely. 



1 On the western coasts of Europe and America, where the change in surface 

 temperature is very gradual, Purpura lapillus (the west American 'species' are 

 at best only derivatives) is able to creep as far south aslat. 82° (Mogador) in the 

 former case, and lat. 24° (Margarita Bay) in the latter, the mean annual tem- 

 perature of the surface water being 66° off Mogador, with an extreme range of 

 only 8°, and that of Margarita Bay 73°, with an extreme range of only 5°. On 

 the eastern coasts, where the Pacific and Atlantic gulf -streams cause a sudden 

 change of temperature, the Purpura is barred back at points many degrees 

 farther north, viz. at lat. 41° (Hakodadi), surface temperature 52°, extreme 

 range 25°; and at lat. 42° (Newhaven), surface temperature 52°, extreme range 30°. 



