PRESENT HISTORY AND DISTRIBUTION 65 
representative in the Island of Curacoa. Many of 
the Mediterranean islands have also each their dis- 
tinctive shells. 
Lakes that have long been isolated similarly 
furnish instances of peculiar faunas. Amongst these 
Lake Tanganyika is the most interesting. Some of 
the species there have such stout shells and present 
such a marine facies that (with the added presence 
of a Jelly-fish and other creatures which it had been 
customary to associate with marine conditions) it is 
not surprising that for many years it was thought 
this lake must formerly have been connected with 
the ocean. That idea, however, is now known to 
be quite without justification, and the fauna to be 
a freshwater one that has acquired its present 
character during a long period of isolation. 
More remarkable instances of localized habitat are 
those of which there are two at least in the British 
Islands. Limnea involuta is only found in asmall tarn 
on a mountain overlooking the Lakes of Killarney, and 
L. Burnetti occurs solely in Loch Skene (Scotland). 
Per contra, some of the freshwater genera, such as 
Limnea, Planorbis, Ancylus, Physa, Vivipara, Theodoxis, 
and Unio, have an almost world-wide distribution. 
On the whole, however, like their marine kindred, 
the non-marine Mollusca are capable of being 
divided according to the prevalence of peculiar forms 
into faunas occupying roughly defined areas. Thirty- 
one “regions,” as they are called, may be recognized, 
and these can also be conveniently grouped accord- 
5 
