M. C. Mereschkowsky on the Hydroida, 335 
Polyserias mirabilis, Verrill. (Pl. XV. figs. 5, 6.) 
Diphasia mirabilis, Vervill, Amer. Journ. Sci. vol. v. (Dec. 1872), 
p- 9; 8.Smith & O. Hagen, Trans. Conn. Acad. of Arts & Sci. vol. iii. 
pt. i. (1877), pp. 219, 225; Clark, Proc. Acad. Phil. 1877, pt. iii. 
p- 219, pl. xiii. fig. 36. 
Polyserias Hincksti, Mereschkowsky, Ann. & Mag. Nat. Hist. vol. xx. 
(1877) p. 228, pl. vi. figs. 15, 16. 
Colony rigid, plume-like, attaining a length of 16 and a 
breadth of 6 centims. The principal stem is angularly bent, 
and only bears two series of hydrothece ; from each angle 
issues a long and straight branch which is never ramified ; 
the arrangement of the branches is regular, alternate, and in 
the same plane; they are of equal length to [near] the ex- 
tremity, where they become shorter. Sometimes, especially 
in the largest colonies, the stem gives off from its two lower 
bends, not, as usual, a single branch, but [twoor more] branches, 
which issue simultaneously from the angle formed by the prin- 
cipal stem ; and in this very rare case each pair of branches is 
not arranged in the same plane as all the other branches. 
Each branch is attached by means of a short peduncle, and 
forms with the principal stem an angle of about 45°. The 
hydrothece upon the branches are always arranged in six dis- 
tinct and regular rows, even to the ends of the branches, 
which terminate abruptly. The transverse section of the 
branch, if it is rather slender, only shows three cells around 
the central cavity ; but on making the section a little higher 
up, we obtain three other cells, placed, not directly above the 
former, but between them in the interstices—which proves 
that we have to do with six rows, and that at the same time 
two hydrothecee belonging to two rows are not placed side by 
side, but sometimes higher, sometimes lower (that is to say, 
alternately). This will be better understood by examining 
the drawing which I have already given*. ‘The form of the 
hydrothece is that of those of the Sertularie in general, fur- 
nished with a pretty long neck inclined outwards and slightly 
flattened, and with a wider part united with the stem. The 
aperture of the hydrotheca is operculate and furnished with 
two very distinct teeth placed at the corners of the orifice, 
which, in consequence of the compression of the neck, is 
elongated. . 
The gonophores in their young state have the form of a 
reversed cone attached by its apex (Pl. XV. fig. 5); but in the 
completely developed state they differ very little from the 
gonophores of Sertwlarta or Thuiaria. Their form is elon- 
* Ann, & Mag. Nat. Mist. 1877, vol. xx. pl. vi. fig. 16. 
