AGLAOPHENIA RAMOSA. 39 
the European shores or P. jilicula, etc. of the present Report, we find that 
the hydrothecal ramuli are given off on two opposite sides of the simple or 
branching stem from which they spring with a regularly alternate arrange- 
ment. In P. catharina the hydrothecal ramuli, instead of being alternate, 
are exactly opposite. In P. geminata, while the points of origin of these 
ramuli are opposite to one another, as in P. catharina, the ramuli themselves 
are all directed to one side, and thus lie in unilateral pairs along the sup- 
porting branch. In the genus Monostwchas, as represented by the single 
species M. dichotoma, the main stem is dichotomously branched, and every al- 
ternate prong of the forks gives off— from one side only — the hydrothecal 
ramuli. In Antennella the whole of the main stem has disappeared, and the 
hydrothecal ramuli come to be borne directly on the hydrorhiza. 
Again, in Antennularia the hydrothecal ramuli are disposed in regard: 
verticils along the stem. In Antennopsis they also surround the stem, 
but instead of being disposed in verticils they are scattered. What An- 
tennularia is to P. catharina with its opposite distichous ramuli, Anten- 
nopsis is to P. setacea with its alternate distichous ramuli. Finally, in 
Hippurella we have a transition between the alternate and distichous 
ramuli of P. setacea and the scattered ramuli of Antennopsis or verticillate 
ramuli of Antennularia; the proximal parts of the branches having their 
hydrothecal ramuli disposed as in P. sefacea, while towards the distal 
extremity these ramuli become scattered as in Antennopsis or verticillate 
as in Antennularia. 
Each of these modifications is in itself so well marked that it may be 
justly taken as characterizing a distinct specific form, or, in some cases, even 
a generic group. Any one of them, however, may be regarded as an easily 
understood derivation from others, while all may obviously have descended 
from a single ancestral form 
Genus AGLAOPHENIA Lamourovx (in part). 
Aglaophenia ramosa. 
Pl. XXIIT. Figs. 1-4. 
Trophosome. — Hydrocaulus attaining a height of about six inches, sub- 
dichotomously branched, fascicled in main stem and branches, and becoming 
monosiphonic only near the distal ends; pinne given off at an acute angle 
from the anterior aspect of the branches; internodes of rachis with a 
