74 M. FOSLIE. [1905 



Lithoth. membranaceum, as a general rule, is easily recogniz- 

 able. It forms filmy patches on the fronds of other algae, parti- 

 cularly Florideae, the colour of the host plant showing through the 

 species in question. The crust is frequently at first orbicular, then 

 many crusts becoming confluent, indefinite in outline. It adheres 

 very closely to the substratum, becomes wrinkled as the latter 

 contracts in drying, but does not crumble and fall from it, as 

 some other epiphytic calcareous algae do, being but little incrusted 

 with carbonate of lime. Sometimes a new crust is here and there 

 developed over the primary one, then the former becomes a little 

 brittle, and if only provided with cystocarpic conceptacles the spe- 

 cies may be confounded with Melobesia Lejolisii which now and 

 then covers L. membranaceum. 



The frond is frequently composed of two to five layers of 

 cells. Only the youngest parts of the frond are in part monostro- 

 matic. In these parts the cells are elongated in the direction of 

 the radius, 9 — 18 p long and 6—8 p broad. In vertical section 

 the basal cells are somewhat varying. Thus when the alga grows 

 e. g. on Furcellaria they have generally their largest length ver- 

 tically, 10 — 18 p long and 7 — 14 p broad, but when growing on 

 Rhodymenia the cells are partly squarish, about 1 1 p in diameter, 

 partly vertically or horizontally elongated, in both the latter cases 

 of about the same size as on Furcellaria. 



The conceptacles of sporangia are convex or subhemispherical, 

 frequently crowded and not sharply defined, 160 — 300 p in dia- 

 meter. The central part of the roof is either not incrusted with 

 carbonate of lime or but scarcely so and semi-transparent, inter- 

 sected with 20 — 30 muciferous canals. This part becomes some- 

 what settled towards maturity, and the dark coloured sporangia 

 are often visible through the thin cell-walls of the roof. Therefore, 

 to the naked eye the conceptacles (or properly the central part of 

 them) look like so many dark specks on the surface of the frond. 

 The sporangia are four-parted, 60—90 p long and 40—60 p broad. 

 The conceptacles of cystocarps are subhemispheric- conical and of 

 about the same diameter as those of sporangia. A small part 

 around the orifice is either not incrusted with carbonate of lime 



