AGLAOPHENIA RAMOSA. 39 



the European shores or P. filicula, 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 hydrotliecal 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 Monostsechas, 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. setacea, 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 Lamourotjx (in part). 



Aglaophenia ramosa. 



PL XXIII. 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 ; pinniB given off at an acute angle 



from the anterior aspect of the branches ; internodes of rachis with a 



