1889.] Mr A. H. Cooke, On the common Dog- Whelk. 13 



November 11, 1889. 



Dr Gaskell in the Chair. 

 The following Communications were made: 



(1) On the Varieties and Geographical Distribution of the com- 

 mon Dog-Whelk {Purpura lapillus L.). By A. H. Cooke, M.A., 

 King's College. 



The author, while unable to advance any satisfactory theory 

 to account for colour variation, held that variations otform largely 

 depended upon the station occupied by the animal. Shells occur- 

 ring in exposed situations (e.g. Land's End, Scilly Islands, coasts 

 of N. Devon and Cornwall) were stunted, with a short spire and 

 large mouth, the latter being developed in order to increase the 

 power of adherence to the rock, and of resistance to wave force. 

 Shells occurring in sheltered situations, estuaries, narrow straits, 

 &c. where there was no severe wave force to encounter, were of 

 great size, spire well developed, mouth small in proportion to 

 area of shell. This view was illustrated by series of specimens 

 collected at various points on the British coasts. 



With regard to the question of geographical distribution it was 

 shewn th&tPurpura lapillus (a "north temperate" species) occurred 

 on the East Asiatic coasts from Behring's Straits to Hakodadi 

 (41°), on West European coasts from North Cape to Mogador 

 (32°), not entering the Mediterranean, on East American coasts 

 from Greenland to Newhaven (42°), and on West American coasts 

 (assuming the identity of the West American Purpuras with 

 lapillus) from Alaska to Margarita Bay (24°). Thus on the two 

 western coasts it had a far more southern range than on the two 

 eastern. The author regarded this fact as due to the direct 

 influence of the surface temperature of the ocean. The mean 

 annual temperature (taken from the Meteorological Society's 

 charts) of the surface water at Hakodadi was 52°, with an extreme 

 range of 25°; that of Mogador was 66°, extreme range only 8°; 

 that of Newhaven was 52°, extreme range 30°; that of Margarita 

 Bay 73°, extreme range only 5°. Violent changes of temperature 

 were fatal to life, zones where such changes occurred acted as 

 barriers to distribution; it was possible on the other hand for an 

 organism to bear a gradual change from cold to extreme heat. 

 On the western coasts of Europe and America the change from 

 cold to heat was very gradual, hence the Purpura had been able 

 to creep as far south as 32° in the one case and 24° in the other; 

 while on the opposite eastern coasts, where the Atlantic and 

 Pacific Gulf-streams caused a sudden change in the temperature 

 of the surface-water, the species was barred back at a point many 

 decrees further north. 



