Ck. XVIII.] LAXD AND FRESH- WATER MOLLUSKS. 333 



ference in the distribution of the land and fresh- 

 water species. According to Mr. Lovell Reeve, who has 

 specially studied this question, out of many hundreds of 

 land mollusks inhabiting the Caucasian province at its 

 centre in Hungary and Austria, only ninety extend to the 

 British Isles, and of these thirty-five do not reach Scot- 

 land. Upwards of two hundred species of Clausilia are 

 to be found in the centre of the province, and of these 

 only four reach Britain, and only one Scotland. Out of 

 five hundred and sixty species of Helix inhabiting the 

 Caucasian province, there are but twenty-four in Britain. 

 Whilst the distribution of the terrestrial mollusks of 

 Europe is thus restricted in range, though the species are 

 numerous, the fresh-water shells are few in species, but 

 of wide distribution. Quoting again from Mr. Reeve : 

 Of the Lymnceacea "there are not six species, it may be 

 safely stated, in all Europe, more than there are in 

 Britain. They have no particular centre of creation. 

 There is no evidence to show whether the alleged pro- 

 genitors of our British species were created in Siberia, 

 Hungary, or Thibet. There is scarcely any variation 

 either in the form, or number of the species in those 

 remote localities. Of Planorbis scarcely more than 

 fifteen species inhabit the whole Caucasian province, and 

 we have eleven of them in Britain." " Of Physa and 

 Lymmea, it is extremely doubtful whether there are any 

 species throughout the province more than we have in 

 Britain. Neither of Ancylus, which lives attached, 

 limpet-Hive, to sticks and stones, and has very limited 

 facilities of migration, are there any species throughout 

 the province more than we have in Britain/' * 



* Lovell Reeve, " British Land and Fresh- Water Mollusks," p. 255. 



