IV INTRODUCTION. 



throughout Northern Europe and Asia, and the sea species on 

 almost every coast of the North Sea and along the eastern half of 

 the North Atlantic. Some are found much further away. lanthina, 

 the most beautiful of our univalves, comes drifting to us across the 

 wide ocean, and is known all the way from Patagonia. 



A mollusc is British if it has been dredged up in British seas 

 or found alive in any part of the British Isles. Thus a good wide 

 net is spread for stragglers, and nationality seems to have been 

 occasionally determined by the arrival of an individual in a gale of 

 wind. This is inevitable, but it should be remembered that most 

 of the mollusca are dioecious, though some are monoecious, and 

 among the bivalves the same genus may have the sexes of its 

 species distinct or combined. For instance, the British oyster is 

 monoecious, but the American oyster is dioecious, and when trans- 

 planted to this country has to have both sexes brought across 

 the Atlantic. 



This oyster is one of several cases of successful acclimatisation, 

 though mollusca are not so easily settled in strange lands as might 

 be supposed. The fact that nineteen of our land shells have taken 

 up their abode in Australia merely shows that a few species are 

 capable of thriving almost anywhere. There are British snails in 

 most ports frequented by British ships ; they travel in the packages 

 and packing materials. Foreigners evidently come here in a similar 

 way, and some of our novelties are traceable to nursery gardens, to 

 which they have been imported in the mould around the roots of 

 the plants. But no matter how they came or how widely they may 

 be distributed over the world, they are British if they are found 

 here, and thus our country's mollusca number over 700 species, 

 with perhaps three times as many varieties. 



W. J. G. 



