M. C. Mereschkowsky on the Hydroida. 325 
Calycella syringa, several Sertularie, Sertularella rugosa and 
tricuspidata proves that the fauna of the White Sea is only a 
special department of a circumpolar fauna. 
From what we now know of the distribution of the Hydroids 
it may be seen that, in fact, there exists such a circumpolar 
fauna, on the one hand perfectly special, and on the other 
represented by species which also occur in Europe, in England, 
the Baltic, &c. It is always easy to recognize to which fauna 
a species must belong, from a consideration of its size: in its 
native place, in the country from which it started, the species 
will certainly appear in all its splendour and of its largest size ; 
for it is there especially that the conditions of life are most 
favourable toit. Thus, among the Hydroids there are certain 
species which frequently occur in the north (Iceland, Green- 
land, Spitzbergen, &c.), and which are there distinguished 
from the same species obtained from England, for example, 
by their excessive size. It is clear, therefore, that the polar 
regions must be regarded as the native place of these species, 
as the starting-point from which they have spread southwards 
into warmer seas, which certainly must have had an effect 
upon them, rendering them feebler; and it is in this that I 
find the answer to the question raised by Hincks*, as to why 
this phenomenon is observed. But, on the other hand, it must 
not be forgotten that in the family Plumulariide there are 
species characteristic of the southern seas of gigantic size, as, 
for example, that described by M. Sempert, which proves 
that the native place and starting-point of all these Hydroids 
must be regarded as in southern regions; and it is very pro- 
bable that the further to the north they are met with, the 
weaker and poorer they will be. 
I will now pass to the descriptions of new Hydroids. 
OoRHIZA, nov. gen. (Pl. XV. figs. 7-11.) 
Hydrorhiza a continuous layer consisting of a mass of 
anastomosing tubes, covering the shells of Gasteropods. From 
its surface rise spines and sexual and nutritive individuals. 
Trophosome cylindrical, with a single whorl of filiform tenta- 
cles. The sporosacs rise directly from the hydrorhiza, with- 
out the intervention of blastostyles. 
As will be seen from the character of this genus, it must 
undoubtedly be placed in the family Hydractiniide, which 
appears at once from the habit of this Hydroid. The con- 
tinuous layer of the hydrorhiza, the spines, and the long and 
* Ann. & Mag. Nat. Hist. 1874, vol. xiii. p. 147. 
+ Zeitschr. fiir wiss. Zool. vol. xiii. p. 560. 
