Mysis relicta, Lovén, in Ireland. 595 
work ‘Cladocera Suecica’ (cf. p. 226 and plates), who also 
has been kind enough to examine specimens for me. 
Further investigation in the year 1900 showed that in 
Lower Lough Erne also Mysis relicta exists in great numbers, 
every haul of the dredge producing a plentiful supply both in 
the shallower waters near the shore and in soundings from 
100 to 180 feet in the middle of the broadest part of the lake, 
here about 5 miles across. Gatherings made about the 1st of 
June revealed the fact that the greater part of those taken 
were young individuals of about 9 millim. in length, mixed 
with a small proportion of adult specimens of full size, namely 
about 22 millim., which is the same as that of the females 
with ovigerous sacs taken in the Upper Lake in November; 
and therefore this may be taken as the average extreme length 
attained by adults in Lough Erne. An examination of these 
revealed very slight variation from the continental forms 
figured by Sars. To indicate the importance of a careful 
comparison of Irish with continental examples it is neces- 
sary to recapitulate the points of divergence between Mysis 
relicta and Mysts oculata. ‘The theory put forward by Lovén 
and generally adopted in reference to the five freshwater 
Crustacea above referred to, which are only distinguished 
from their marine congeners by very trivial distinctions, is 
that the large lakes in which they are found were in late 
geological times beneath the sea-level, but by the elevation 
of the sea-bottom were converted into basins whose large 
extent ensured a very slow alteration of their contents by a 
very gradual reduction of their salinity. Those Crustacea 
which have survived proved able to accustom themselves to 
the gradual disappearance of the saline constituents of their 
habitat. And in process of time they appear to have deve- 
loped the slight alterations which now distinguish them from 
the marine type, partly influenced perhaps by the altered 
medium and possibly in some respects by isolation. The 
chiet differentiating characters between Myszs relicta and the 
larger Mysts oculata as given by Czerniavski and others may 
be stated as follows:—The flat appendix or scale of the 
second pair of antennae in MM. relicta exceeds by only one 
third the peduncle of the first pair, being proportionally 
shorter and broader than that of J. oculata. ‘The armature 
ot the mandibles is more complicated. The segments of the 
first pair of maxillipeds are shorter. The tarsi of the thoracic 
feet comprise only from six to eight segments, not nine as in 
the adult VW. oculata (according to Kroyer). The third and 
fourth pair of the pleopods of the male, though agreeing in 
