xii WIDE DISTRIBUTION OF DEEP WATER FORMS 363 



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

 of the Eio de la Plata; Semele jj'^^'rfundorum in 1125 fath. 

 near the Canaries, and in 2900 fath. mid iST. Pacific; Verti- 

 cordia dcshayesiana in 155 fath. near Cape York, and in 350 

 fath. off Pernambuco ; Area ])teroessa in 2050 fath. mid IST. 

 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 Fernandez; 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 

 surprising range was occasionally found even in shallow-water 

 species ; thus Fetricola lapieida was discovered by the same 

 expedition in the West Indies and N. Australia, Cardita eahjcu- 

 lata off Teneriffe and in Bass Strait, Area imhrieata off Cape 

 York and in the West Indies, Modiolaria euneata at Port Jack- 

 son and Cape of Good Hope, Lima sqiiamosa 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 MoUusca under Eegions and Provinces can only 

 apply to Mollusca which occur at moderate depths. The most 

 important factor in the environment, as determining distribution, 

 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 tem- 

 perature 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. 



^ Oil tlie western coasts of Europe aiicl America, where the change in surface 

 temperature is very gradual, Fur-pura lapillus (the west American ' species ' are 

 at best only derivatives) is able to creep as far south as lat. 32' (Mogador) in the 

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

 ture of the surface -water being 66' off Mogador, with an extreme range of only 8', 

 and that of Mai-garita 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 tem- 

 l)erature, the Purpura is barred back at points many degrees farther north, viz. at 

 lat. 41° (Hakodadi), surface temperature r)2°, extreme range 25° ; and at lat. 42° 

 (Newhaven), surface temperature 52°, extreme range 30°. 



