140 COLLINS AND HERVEY. 



Sound, Jan., cave, Agar's Island, July, Aug., shore of Gibbet Island, 

 Aug., cave, Ducking Stool, Dec., Collins. Tetraspores in April, Aug. 

 and Dec. 



Growing on walls and roofs of caves, or in smaller cavities in rocks, 

 usually among other algae with a creeping base, such as Rhodochorton 

 and Spermothamnion. We have found this quite a puzzling form, as 

 there are three European species, in different genera, of similar size 

 and habit, and with tetraspores similarly placed. Two of these, 

 Ptilothamnion pluma Bornet in LeJolis, 1863, p. 118 and Plumaria 

 Schousboei (Bornet) Schmitz, 1896, p. 7, while differing in cystocarps 

 and antheridia, are almost identical otherwise, but a comparison of 

 the plates, Bornet & Thuret, 1876, p. 32, PL X, and Bornet & Thuret, 

 1880, p. 179, PI. XLVI, shows that the Plumaria has shorter cells 

 throughout, and that the branches arise at the upper edge of the cells 

 of the axis, while in the Ptilothamnion they are distinctly lower. 

 The Bermuda plant agrees exactly with Plumaria Schousboei in form 

 and proportions of cells, and place of insertion of branches; on that 

 account we have placed it in the genus Gymnothamnion, recognizing, 

 however, that when the sexual fruit is discovered, a different disposi- 

 tion may be necessary. The uniformly opposite character of the 

 branching is quite noticeable; even in the prostrate filaments any 

 other branching is rare and evidently abnormal, though in most 

 species with opposite branching in the erect parts, the basal part is 

 apt to vary from this. Moreover, every rhizoid that we have observed 

 is opposite to an erect axis. Bornet and Thuret, describing P. Schous- 

 boei, say " ils adherent par des crampons semblables a ceux des Spermo- 

 thamnion et des Polysiphonia"; but an examination of PI. X, fig. 1, 

 shows all the rhizoids opposite to erect axes, while this is not the case 

 in Spermothamnion flabellatum shown in PI. VIII, fig. 1. We are led 

 to keep the Bermuda plant separate from P. Schousboei principally 

 from the fact that all well developed fronds bear erect branches of a 

 second order, in luxuriant individuals one such branch from every 

 cell of a branch of the first order, always on the upper side of the 

 latter. If these branches were in pairs, the distinction would be less 

 important, and might mean merely a repetition of the normal branch- 

 ing in luxuriant individuals, but we have never seen an outgrowth 

 from the under side of a branch of the first order. Curiously enough, 

 this mode of branching is identical with that of Antithamnion ptero- 

 ton (Schousb.) Bornet, 1892, p. 331, PI. Ill, figs. 8-9, in regard to 

 which the author says, " Cette elegante petite Algue ressemble beau- 

 coup aux Callithamnion pluma et elegans mais elle est plus delicate, 



