108 PKOCKKDINGS OK THK M A LACOLOCJICA I, .SOCIKTY. 



Again, certain nortliern species wliicli are littoral under normal 

 conditions of life tend to seek deeper water as they move southward, 

 while exactly the reverse is the case with certain southern species, 

 which, are found in shallower water in northern than in southern 

 latitudes.' This seems to emphasize the point that temperature is of 

 prime importance in determining habitat, northern species finding the 

 cold they are accustomed to by migrating to somewhat deeper water 

 in the soutli, and southern deepish water species finding the low 

 temperature they need in shallower water as they move north. 

 Mediterranean species of the coralline and abyssal fauna are found to 

 live, as a rule, in depths less profound than they do in the Atlantic. 

 Bucchmm nndatiim, L., wliich is common at low-water mark at certain 

 places in Northern and Eastern England, is never found between tide- 

 marks at Scilly. Neptimea antiqua, L., may be found alive on the 

 shores of Shetland, but in Southern and Western England it retires 

 into deeper water. Emarginula crassa, J. Sow., is not rare on the 

 shore at Oban; in the Gulf of Gascony it has only been dredged at 

 400-500 metres. 



It must not be forgotten that conditions of life in every (juarter of 

 tlie globe can never be regarded as absolutely permanent. Ctiauges 

 of environment, some vast and sweeping, others apparently trivial and 

 scarcely detected, are in operation and must affect, to a greater or less 

 degree, the life of the organisms which inhabit the different areas. 

 Collectors who work a particular ground are familiar with the fact 

 that certain species may be found by the score or by the hundred in 

 a given locality, and then for years they will be extremely scarce, 

 and then will reappear again, as numerous as before. Of Aplysia 

 depihns, L., "a small fleet arrived in Torbay in 1875 and lingered 

 for a couple of years . . . previous to that only one specimen had 

 been found there." Of Oscanius memhranace^is, Mont., "in 1874 

 a large fleet appeared simultaneously at Weymouth and at Torbay, 

 and again in the latter district in 1877 and 1887." - "At one time 

 Nassa fossafa, Gld., at another Periploma discus, Stearns, at another 

 Zima orientaUs, Cpr., or Scalatella striata, Cpr., are found by the 

 dozen in San Pedro Bay [Cal.], and then for years after only a few 

 are found at a time." ^ It must be remembered too that certain 

 Mollusca, notably the Opisthobrancliia and Nudibranchia, come ashore 

 in the breeding season to deposit tlieir eggs and then retire to deep 

 water. 



Occasionally we are able to observe a definite extension of area on 

 the part of a species, without being able to assign any definite cause. 

 When Jeffreys wrote his British Conchology ( 1 865 is the date of vol. iii) 

 Aoncea testiidinalis, Miill., had not been observed on our eastern coast 

 south of Hartlepool; in 1890 it had reached Scarborough, in 1910 it 

 was south of Bridlington, and is said to be extending its range rapidly 



1 See W. H. Dall, Bull. U.S. Nat. Mus., vol. xxxvii, pp. 1-221, 1889. 

 ^ J. T. Marshal], Journ. Conch., vol. xiv, pp. 65, 66, 1913. 



=' S. P. Monks, Nautilus, vol. vii, p. 75, 1893. 



