64 THE LIFE OF THE MOLLUSCA 



Plicatula, Crassatella, Tridacna, Sanguinolaria, and 

 Nautilus, are more peculiar to the warmer waters. 



Contrary to what has been asserted, there is no 

 identity between Arctic and Antarctic Mollusca, 

 although the prevailing dull appearance characteristic 

 of Mollusca inhabiting cold waters, as it also is of 

 abyssal kinds, may have helped to foster an imaginary 

 resemblance and give colour to the bipolar theory. 

 Tropical Mollusca, on the other hand, are more 

 highly coloured, some quite brilliantly so. 



The non-marine Mollusca, having far fewer op- 

 portunities for individual dissemination than the 

 marine, whose free-swimming fry are readily trans- 

 ported long distances by currents, have naturally 

 tended to differentiate to a far greater extent. The 

 principal barriers to their dispersal are mountain 

 ranges, deserts, and the sea. Each island of any 

 antiquity offers its own little assemblage, and fre- 

 quently its individual representative. Thyrophorella 

 (ante, p. 23, Plate VIII., Figs. 18 and 19) is con- 

 fined to the Island of San Thome, in the Gulf of 

 Guinea; the genus Achatinella to Ochu, one of the 

 Sandwich Islands, though it is doubtful if, as stated, 

 each valley has its peculiar species; while closely 

 allied forms occur on the neighbouring islands of the 

 group. Cerion is a well-known West Indian genus, 

 of which in the main each species is confined to some 

 single island or group of adjacent islets, and of these 

 shells the first described, and only species known to 

 Linnaeus, C. uva (Plate XIIL, Fig. 17), is the sole 



