372 Marine Algce of Berwloh-un-Tweed. 



4G. Midrib present. ... ... ... ... ... Fucus. 



Midrib wanting. .. ... ... ... ... 47 



47. Fronds destitute of air-bladders. ... ... .. ... 48 



Fronds furnished with air-bladders. .. ... ... 49 



48. Fronds thick and coriaceous, channelled, fruit in terminal receptacles. 



Pelvetia. 

 Fronds thin, membranaceous, dichotomously branched, spores external, 

 scattered singly or in spots over the surface of the frond. Bidyota. 



49. Air-bladders, simple, immersed in the frond, fruit on short, lateral 



branches. ... .. ... ... ... Ascojjhyllum. 



Air-bladders stalked, with numerous diaphragms, fruit terminal. 



Halidrys. 



50. Fronds calcareous. ... .. ... ... ... 51 



Fronds not calcareous. ... ... ... ... ... 53 



51. Fronds erect, filiform, articulated. . ... Corallina. 



Fronds thin, horizontally expanded. ... ... ... 52 



Fronds thick, horizontally expanded, but rising at intervals into 

 irregular protuberances. ... ... ... Lithothamnion. 



52. Fronds semi-transparent, composed of a single layer of cells. 



Hapalidium. 

 Fronds opaque, composed of more than one layer of cells. Melobesia. 



53. Fronds horizontally expanded, crustaceous or membranaceous. 54 



Fronds erect or umbilicate. ... ... ... ... 57 



Plant parasitic on Rhodomela, forming irregularly shaped convex 

 masses on the stem of the host-plant. ... .■ Harreyella. 



54. Fronds cartilagino-membranaceous or membranaceous. ... 55 



Fronds gelathio-coriaceous. ... ... ... .. 56 



55. Spores in external warts (nemathecia) composed of slender parallel 



filaments on which are borne the antheridea, cystocarps, and 

 tetraspores, nemathecial filaments united by a gelatinous substance, 

 tetraspores cruciate. ... ... .. ... Peyssonnelia. 



Spores accompanied by free, curved, stiff paraphyses, in external 

 spots 'sori), tetraspores cruciate. ... ... ... Bhododermis 



Spores in cavities sunk in the frond, tetraspores zonate. 



Hildenbrandtia. 

 Spores in external nemathecia, tetraspores zonate. Hcematnphlcea. 



56. Fronds parenchymatous below, but above formed of looselyunited simple 



filaments, tetraspores cruciate, formed in the filaments. Petrocelis. 

 Fronds formed of loosely united, simple or slightly branched filaments, 

 tetraspores lateral, zonate. ... ... ... Cruoria. 



Fronds composed or loosely united, dichotomously branched filaments, 

 bent horizontally below. ... .. .. Eoematocelis. 



57. Fronds tubular. ... ... ... ... ... 58 



Fronds filamentous or slightly compressed. ... ... 59 



Fronds membranaceous. ... ... ... ... 73 



58. Fronds brownish-purple, thick, soft, hollow, fruit immersed in the 

 frond, tetraspores cruciate. ... .. ... Dumonlia, 



