Mysis relicta, Lovén, in Ireland. 397 
the tarsi of the thoracic feet were composed of from seven to 
eight segments. ‘he contour of the indent of the telson 
corresponds very nearly with the figure given by Sars, the 
convexity at the base of the terminal spines being very 
moderate. 
If the generally accepted theory is correct, namely, that 
Mysis oculata was introduced into freshwater lakes by the 
Glacial sea when subsidence of portions of the present land- 
surface took place, no difficulty arises as to its introduction 
thus into Lough Neagh, for geologists agree as to the sub- 
mergence of a part of the east of Ireland as well as the central 
plain. A subsidence of only about 10 feet now would bring 
the floor of the greater portion of that lake to the level of the 
sea. But with regard to Lough Erne there is some room for 
doubt whether the introduction can have taken place from 
the Atlantic seaboard, because although its proximity would 
suggest that to be probable, and its present surface-level is 
only about 180 feet above the sea at high-water spring-tides, 
while a considerable area gives soundings of between 100 to 
200 feet in depth, one spot reaching about 220 feet, yet 
geological maps do not give the Glacial sea of Pleistocene 
times such a western extension, the western seaboard of 
Treland being supposed by many authorities to have been 
tilted up, except where the central plain reaches Galway Bay. 
But since the large basin occupied by Lough Oughter in 
Co. Cavan, now about 250 feet above sea-level, is included 
in the eastern area referred to which was submerged, it is 
very probable that marine organisms would have found their 
way thereto, as in the case of Lough Neagh. With an 
elevation of the west coast the drainage of the Erne district 
would have then flowed easterly ; but when the land-surface 
attained its present relative level, and the transgression of the 
sea over the eastern area ceased to exist, and the watershed 
became changed, the drainage of Lough Oughter flowed west- 
ward into Upper Lough Erne as at present, and such marine 
organisms as survived would have had an opportunity of 
colonizing its waters. 
I hope at an early date to explore a portion of Lough 
Oughter with the object of finding Mysis relicta. I desire to 
express my obligations to the Rev. Canon Norman for access 
to his valuable library and other assistance. 
Drumreaske House, Monaghan. 
