166 HISTORY OF THE EUROPEAN FAUNA. 



the Arctic Regions and survive in isolated patches in 

 favourable localities. We find seven species of these 

 American plants in Ireland, almost entirely confined 

 to the north and west coasts. These are Spiranthes 

 Romanzoviana, Sisyrinchium anceps, Naias flexilis, 

 Eriocaulon septangular e^Juncus tennis , and Polygonum 

 sagittifoliuui. To them must be added another plant 

 recently discovered by the Rev. Mr. Marshall in the 

 south of Ireland, namely Sisyrinchium calif or nicum. 

 As I have mentioned in former writings, there are 

 three species of North American freshwater-sponges 

 in Ireland which have not hitherto been discovered 

 elsewhere in Europe or in Asia. These, namely 

 Ephydatia crater if ormis, Heteromeyenia Ryderi, and 

 Tubella pennsylvanica, all occur in some of the lakes 

 near the western coast of Ireland. 



There are in all groups of animals instances of species 

 which are confined to Europe and North America, while 

 unknown from the Asiatic continent, but none, as far 

 as is known, have such a very discontinuous range as 

 that of the animals and plants just referred to. In 

 some cases the species still occur in Greenland, and 

 in this way make it still clearer that their migration 

 in former times took place from one continent to 

 the other by way of that country. As an interesting 

 instance of such distribution may be mentioned the 

 Common Stickleback (Gasterostens aculeatus], which 

 is found in Greenland, North America, and Europe, 

 but is quite absent from Asia. Then again, the Nine- 

 spined Stickleback (Gasterosteus pungitius] is confined 



