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 Thomé, 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 
Linnzus, C. wa (Plate XIII., Fig. 17), is the sole 
