172 



placed together with specimens of the last named species, of 

 which, on the other hand, no specimen is to be found from that 

 locality. 1 ) 



Description of the species. The named specimens are much 

 bleached, with a faint yellowish tinge, orbicular or nearly orbicular, 

 3—6 mm. in diameter, occasionally somewhat confluent, and clo- 

 sely adherent to -the substratum. They are in the central portion 

 scarcely more than 0.2 mm. thick. The nature of the surface - 

 appears to be determined by that of the substratum, most often 

 nearly smooth, or the unevenness partly caused by scars after 

 emptied conceptacles, which are gradually filled with new formed 

 tissue, but the latter not always regularly effacing the scars. A 

 broader or narrower part of the peripherical portion is very thin, 

 shallowly undulate-crenate, and is frequently rather distinctly mar- 

 ked from the internal and thicker part of the crust. Pl. 22, fig. 10. 



The cells of the upper thickening layer are, so far as I have 

 seen, rectangular with rounded corners, about 7 — 10 p. long and 

 5 — 6 p. thick with thin walls. 



The conceptacles of sporangia never grow down into the 

 frond, so far as may be judged from the scanty materials at my 

 disposal. They are small and somewhat crowded in the central 

 portion of the frond, seen from the surface about 250 fi in dia- 

 meter, and the circular, slightly convex roof very little prominent, 

 partly and more often not or scarcely not raised above the sur- 

 face of the frond, but distinct and nearly always surrounded by 

 an annular, shallow deepening. Pl. 22, fig. 11. The roof falling 

 away a shallow or deeper scar after the emptied conceptacles 

 appears on the surface of the frond, which by and by becomes 

 filled by new formed tissue. The only conceptacle examined was 

 not provided with mature sporangia, a solitary one nearly linear, 

 about 80 ;j. long and 20 fi broad, with a founded but very indi- 

 stinct partition from the periphery towards the middle of the cell. 

 I, therefore, cannot decide whether the sporangia are bisporic or 

 tetrasporic. 



I got a Lithothamnion from Mr. Ba t ters, gathered at Arran, 



*} Cp. Are sch. 1. c. and Ekman, Bidr. p. 5. 



