MONOSTASCHAS DICHOTOMA. 37 
with entire margin, associated with a pair of supracalycine nematophores, 
and with free mesial nematophores. 
Gonosome. — Gonangia not contained in corbule, or connected with special 
branches. 
The unilateral disposition of the hydrothecal or ultimate ramuli on the 
main branches is very remarkable, and, being absolutely constant, becomes 
a character of generic value. 
Monostzchas dichotoma. 
Pl. XXII. Figs. 1-5. 
Trophosome. — Hydrocaulus attaining a height of about an inch and a half, 
pellucid, dichotomously branched ; every alternate internode of the hydro- 
thecal ramuli carrying a hydrotheca and separated from the others by a 
transverse distal joint, and a very oblique and more strongly marked 
proximal one. Hydrothece rather large cup-shaped, flanked on each 
side by a prominent tooth-like process of the internode. Supracalycine 
nematophores borne by the tooth-like processes, one mesial nematophore 
borne by the hydrotheca-bearing internode at the proximal side of the 
hydrotheca, and two by the intervening internode; numerous nemato- 
phores borne in a single series along the opposed sides of the bifurcat- 
ing branches. 
Gonosome. — Gonangia pyriform, contracted below into a short stalk, which 
springs from a slightly prominent process of the internode, just below the 
base of the hydrotheca, and carries a nematophore on each side of it. 
Off Pacific Reef, from a depth of 283 fathoms. 
The present species closely resembles, in several important characters, 
the Plumularia catharina of Johnston. In the form and position of the 
hydrothece, in the internodes and nematophores of the ultimate ramuli, 
and in the form and position of the gonangia, the resemblance is so close, 
even in minute details, that it is impossible to find in these parts any 
characters by which the one hydroid can be distinguished from the other. 
It is entirely different, however, with the ramification of the species for 
which I have here founded the genus Monostacuas. This ramification is 
of a very remarkable kind, so much so, indeed, that I regard it as affording 
a character of generic value. It has not only no resemblance to that of 
Plumularia catharina, but belongs to a type which has no representative 
in any other known Plumularidan, 
