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. Limncea involuta is only found in a small tarn 

 on a mountain overlooking the Lakes of Killarney, and 

 L. Bavnetti occurs solely in Loch Skene (Scotland). 

 Per contra, some of the freshwater genera, such as 

 Limncea, 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 



