84 Proceedings of the Royal Irish Academy. 



is no doubt that Mysis rdicta of the Great Lakes in America bears the same 

 characters as M. relicta in Ireland and Germany. 



Wesenberg Lund has investigated the Mi/sis relicta of the Fureso lake in 

 Denmark, and finds the presence of this organism is to be accounted for in a 

 similar way to that supplied by the theory put forward by Samter and 

 Weltner for the lakes in north Germany. In the latter case the distribution 

 is intimately bound up with the glaciation of northern Europe. As the ice 

 disappeared, the present Baltic Sea area was flooded by a salt icy sea flowing 

 in from the north-east by the White Sea, and later communicating with the 

 North Sea. This was the " Yoldia Sea " (so called from deposits of the mollusc 

 Yoldia), and in it lived a Schizopod — Mi/sis ocidata. 



As the land was elevated, the Yoldia Sea became more and more land- 

 locked, and was then gradually freshened by melting ice, until an inland 

 fresh-water sea — the Ancyclus Sea — took its place. In the course of this 

 change the arctic Mysis oculata was slowly converted into Mysis relicta. 

 Finally, owing to another communication with the North Sea, salt water 

 streamed into the Ancyclus Sea, bringing in the marine fauna now present in 

 the Baltic. Mysis relicta, however, fled before the invading salt water up the 

 rivers into the lakes of north Germany, where it is now found. This was the 

 sequence of events in Europe. Obviously, for those lakes like Lough Neagh 

 and the Great Lakes of North America, other methods may have been 

 adopted ; and the derivation of the same form is due to the same conditions 

 acting upon the widely distributed Mysis oculata. Now, so far as Lough 

 Neagh is concerned, we see that one of two methods might have been in play. 

 Either Lough Neagh was connected with the sea, was an arm of the sea, and 

 flooded with salt water, in which Mysis oculata lived, or else Mysis oculata 

 migrated actively up the river, and was slowly converted into Mysis oculata 

 var. relicta. 



The two forms are very similar, for Sars remarks that the differences 

 between M. oculata and M. relicta vanish if we compare the adult individuals 

 of one with specimens of the other that have not attained full development. 

 In other words, young M. oculata are like adult M relicta. How are we to 

 decide the origin of M. relicta in Lough Neagh ? The details recorded show 

 that it is simply impossible to state that any lake was an arm of the sea merely 

 because certain marine modified forms occur. The decision lies in the hands 

 of the geologist ; and we have only to go back to the end of the Glacial period, 

 Dwerryhouse states,' " a most interesting stage being reached when the Antrim 

 Plateau was free from ice, except along its seaward margin. The valleys of 

 the Bann and Lagan were closed near their mouths; and the then more 



'Dwerryhouse. British Association Eeport, 19U. 



