CHAP. XII PHENOMENA OF DISTRIBUTION 36 1 



which they occur. Thus we are enabled to distinguish Mollusca 

 of (ji) the littoral^ (5) the laminarian^ (/) the nullipore^ or 

 coralline^ and (c?) 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. 



The difficulty of assigning special genera or species to special 

 'zones of depth' is increased by two important facts in the 

 phenomena of distribution. In the first place, it is found that 

 species which occur in shallow water in northern seas often 

 extend to very deep water in much lower latitudes. This in- 

 teresting fact, which shows the importance of temperature in 

 determining distribution, was first established by the dredgings 

 of the Lightning and Porcupine off the western coasts of Europe. 

 In the second place, a certain number of species seem equally at 

 home in shallow and in abyssal waters, in cases where a great 

 difference of latitude does not occur to equalise the temperature. 

 Thus the Challenger found Venus mesodesma living on the beach 

 (New Zealand) and at 1000 fath. (Tristan da Cunha) ; Lima 

 multicostata in ' shallow water ' (Tonga and Port Jackson) and 

 at 1075 fath. (Bermuda); Scalaria acus from 49 to 1254 fath. 

 (N. Atlantic); and S. hellenica from 40 to 1260 fath. (Canaries). 

 The Lightning and Porcupine found, or record as found,^ Anomia 

 ephippium at to 1450 fath., Pecten groenlandicus at 5 to 1785 

 fath., Lima suhauriculata at 10 to 1785 fath., Modiolaria discors 

 at to 1785 fath., Crenella decussata at to 1750 fath., 

 Bacrydium vitreum at 30 to 2750 fath.. Area glacialis at 25 

 to 1620 fath., Astarte compressa at 3 to 2000 fath., and Scro- 

 hicularia longicallus at 20 to 2435 fath. Puneturella noachina 

 has been found at 20 to 1095 fath., Natica groenlandica at 2 to 

 1290 fath., Rissoa tenuisculpta at 25 to 1095 fath. In many 

 of these cases we are assured that no appreciable difference can 

 be detected between specimens from the two extremes of 

 depth. 



In spite, however, of these remarkable vagaries on the part 

 of certain species, we are enabled roughly to distinguish a large 



1 See papers in P. Z. 8. 1878-85. 



