332 M. C. Mereschkowsky on the Hydroida. 
layer is furnished with small spines. The principal stem is 
very wide; it gives origin to slenderer branches, arranged 
alternately and regularly in a single plane, so that the whole 
acquires the aspect of a feather. The lateral branches may 
divide dichotomously at their extremities. The position of 
the hydrothece is not exactly opposite ; their form is not very 
characteristic ; the summit is a little compressed and notched, 
so as to form two points. 
Gonophores unknown. 
Locality.—The narrow part of the White Sea (Gorlo), be- 
tween the river Ponoy and the island of Morjowetz, in 66° 
55' N. lat. and 40° 45’ E. long., at a depth of 20 fathoms, on 
a gravelly bottom (June 28, 1876)? I am not quite sure 
that the ticket attached to this Hydroid is the right cne. 
This is undoubtedly one of the most singular and interest- 
ing species of the genus Sertularia; and, indeed, if the differ- 
ences presented by the hydrorhiza are increased by those of 
the gonophores, it will be necessary to form a distinct genus 
for it. What most characterizes it is the hydrorhiza, which 
is composed ofa rather thin layer, giving origin at its surface 
to several colonies in the form of pretty bushes, so that the 
whole resembles a little shady thicket. Under the microscope 
it is seen that the hydrorhiza is adorned with an irregular 
branching pattern, formed by partitions which are nothing 
but the lateral walls of the tubes of the hydrorhiza, which has 
become a continuous layer by means of these walls (Pl. XIV. 
fig. 5). This is the reason why we always notice that the 
partitions seen in profile have a line in the middle, which is 
caused by these partitions being formed by two lateral walls 
belonging to two neighbouring tubes, which are thus united. 
Both the upper and lower layers of chitine, between which 
the partitions are placed, are nothing but the upper and lower 
walls of the original tubes. ‘hus we see that the hydrorhiza 
is formed of several ramified tubes, which have become fused 
together by their lateral walls; these lateral walls, after 
having joined in. growing, form the pattern already men- 
tioned (figs. 5, 6), whilst the upper and lower walls constitute 
the upper and lower continuous membranes, between which 
the pattern is placed. There are spots at the margin of the 
hydrorhiza where this process is still continuing; and here it 
may all be seen perfectly. 
So far as I know, there is not a single species, not only in 
the whole family Sertulariide, but generally among the 'The- 
caphora, that has a hydrorhiza of this kind, which much re- 
sembles the hydrorhiza of Hydractinia or Podocoryne, but with 
the difference that in these latter the hydrorhiza is composed 
