M. C. Mereschkowsky on the Hydrotda. 333 
of several layers superposed upon one another, whilst in Sertu- 
laria albimaris it only consists of a single layer. 
As I have already said, the surface of the hydrorhiza bears 
processes of chitine in the form of long, slender cones, empty 
1n the middle and without openings at the extremity (fig. 5, a). 
The length of these conical spines does not exceed 0°2 millim. 
These cones, which remind us of the spines in Hydractinia 
and Podocoryne, are not numerous. It is a very mteresting 
fact that, in all the cases in which the hydrorhiza assumes the 
form of a continuous layer, this peculiarity is always com- 
bined with another, namely the existence of spines; and it 
would be interesting to ascertain the wherefore of this charac- 
teristic comcidence that exists between these two facts. 
Another peculiarity presented by this Hydroid is that the 
principal stem (fig. 3) is very wide in one direction (it is 
compressed) ; and this width is not induced by the size or 
breadth of the hydrothecee, but by the central portion which 
bears the hydrothecee, which gives the colony a very peculiar 
habit. The width of the lateral branches is much less. The 
hydrothece are a little compressed at the end, not, however, in 
the same direction as the principal stem, but in a direction 
perpendicular to this ; and their orifice is notched so as to form 
two teeth. In general form they remind us of those of Poly- 
serias mirabilis. Sometimes, however, hydrothece occur with 
very elongated necks bent to one side. ‘Two or three 
pairs of hydrothece (sometimes, especially on the principal 
stem, a single pair) form an articulation which may easily 
be detached. ‘The position of the hydrothece is more or 
less opposite, more alternate on the lateral branches than 
on the principal stem; but even then it is easy to group them 
in pairs; so that, according to M. Kirchenpauer, it would be 
necessary to arrange this species in the genus Dynamena; 
but, considering the insignificance and the want of clearness 
of this distinction, I prefer to retain the English termi- 
nology. aes 
Length of the colony 16 millims.; breadth of the principal 
stem (measured between the outermost summits of two oppo- 
site hydrothece) 0°8, the same breadth in the lateral branches 
0°60-0°73 ; length of the hydrothece 0°43, their breadth 0°17 ; 
length of the spines 0:2. 
POLYSERIAS, nov. gen. 
This genus, belonging to the family Sertulariide, forms a 
very peculiar type among the Hydroids of the order Theca- 
phora, by reason of the arrangement of its hydrothece. Ex- 
cept Salacia abietina and Campanularia verticillata, there 
