Circumpolar Distribution of certain Hydrozoa. 189 
From the preceding it appears that the physiological con- 
ditions of the oyster, and especially its power of propagation, 
may be changed or checked through modifications of circum- 
stances, such as admixture of fresh water or greater tranquility 
of the water, in respect of which there may be notable differ- 
ences between localities situated at short distances from one 
another ; and it is evident that this circumstance must act as a 
bar to its diffusion over a wider area, particularly because it 
is combined with this other peculiarity, that the brood of 
oysters is capable of surviving, freely drifting about, only for 
a short time after having left the mother; it must sink to the 
bottom after a certain time, and remain where it sinks, whether 
the place is favourable for its development or not. A very 
gradual modification of the kind indicated occurs in the water 
of the Kattegat, where the salt water of the North Sea meets 
and is gradually mixed with the fresher water of the Baltic ; 
and accordingly the oyster becomes more and more rare 
as we proceed southwards. At the entrance to the Sound and 
the Belts the species ceases to occur, though the water is not 
fresher than it might survive and even thrive in; but it could 
not propagate there ; and the nearest place where the physical 
conditions of the water permit it to do so, viz. the Bay of 
Aalbeek, just south of the Skaw, is so far away that the spat, 
drifting with the current, must, as a rule, sink before it arrives 
so far. Between these banks and the southern limits of the 
oysters in the Kattegat they occur only seated on large stones, 
singly or rarely three or four together. These scattered indivi- 
duals are often large and fat, but they are barren. 
With regard to parks for fattening oysters the main result 
of these considerations is, that they may be established in places 
where the water is much less salt than on the natural banks, 
if otherwise the conditions are favourable, as to temperature, 
quality of the bottom, quantity of food, &c.; but they cannot 
be made self-supporting. If artificial banks are to be self- 
supporting the water must not be much less salt than on the 
natural banks from which they are stocked. 
XXIII.—Note on Selaginopsis (= Polyserias Hincksii, Mere- 
schkowsky), and on the Circumpolar Distribution of certain 
Hydrozoa. By the Rev. A. M. Norman, M.A. 
THE Polysertas Hincksii of Mereschkowsky, recently figured 
in the ‘ Annals’ (ser. 4, vol. xx. pl. vi. figs. 15, 16), from 
the White Sea, is, I think, unquestionably identical with 
