V. RHODOMELACEiE. 11 



this continent, and most are widely scattered. Amansia, Alsidium, Acanthophora 

 and Digenia are either tropical or subtropical. Odonthalia is confined to the 

 colder regions of the Atlantic and Pacific, and to the circum-polar Ocean. Per- 

 haps its most southern limit in Europe is the Isle of Man (lat. 54° 20'), while in 

 America it extends from the Arctic Coast at least to Halifax Harbour (lat. 44° 35'). 

 Rhodomela is distributed in high northern and southern latitudes. There is a 

 species {R. concinna, Hook, and Harv.) at the southern extremity of the New 

 Zealand gi'oup, closely related to R. jioccosa of our north-west coast ; and others 

 occur at the Falkland Islands and Cape Horn. Rytiphlcea is for the most part sub- 

 tropical, but some of its species on the West Coast of Ireland reach to lat. 55°. 

 Polysij)honia and Dasya may be said to be cosmopolitan. Of the fonner, between 

 two and three hundred species have been described, pi-obably a third too 

 many. Several of these are found in the Eastern and Western Hemispheres, as 

 well as in parallel climates north and south of the line. P. fastigiata is a very 

 constant parasite on Fucus nodosus. wherever that grows. P. nigrescens, under one 

 or other of its forms, prevails along the Atlantic shores ; and P. urceolata and 

 formo.sa are equally spread abroad. Dasya elegans, so abundant from Long Island 

 Sound to Florida, has an equally wide range in Southern Europe ; and a closely 

 allied species, D. villosa, abounds in Van Dieman's Land. It is remarkable that 

 D. cocc'mea so common on the shores of Europe has not yet been found in America, 

 and that the North American s})ecies most nearly related to it has been brought 

 from Puget's Sound, on the Pacific coast. 



STNOPSIS OF THE NORTH ASIERICAN GENERA. 



* Frond Jlat, pinnatijid. 



I. Amansia. Translucent, distinctly midribbed, membranaceous ; the membrane 



formed of large, hexagonal cells, disposed in transverse rows. 



II. Odonthalia. Opaque, obsoletely midribbed, thickish, composed of minute 



cells ; those of the surface very minute, irregularly polygonal. 



** Frond filiform, inarticulate ; the surface cells minute, of irregular form, and 

 irregularly placed (not in transverse lines). Ramuli similar to the rest 

 of the frond in structure. 



III. Alsidium. Laxly cellular ; the ramuli short, spinelike, transversely striate. 

 Stichidia axillaiy, tufted, fusiform. 



lY. Acanthophora. Laxly cellular; the ra»m/i" spinelike, opaque. Stichidia (foimed 

 in swollen ramuli) subglobose, thorny, sessile. 



C2 



