140 



appeai-s not to be uncommon on the British coast, and at Helgo- 

 land it apparently is one of the most common Lithothamnia. An 

 American specimen is gathered at Kennebunkport, Maine. Collin's 

 coll. E. PI. 19, fig. 21 and 23 represent fragments of specimens 

 from Helgoland, and fig. 22 is a younger Norwegian one. 



Description of the species. The plant forms more or less 

 extended crusts on stones and rocks. Several crusts founded on 

 one and the same substratum get fully confluent. It is at first 

 closely adherent to the substratum, but when older it frequently 

 is more or less easily detached from it. The frond is in the cen- 

 tral portions 0.3—0.8 mm. thick, frequently about 0.5 mm., plainly 

 decreasing in thickness towards the margin, and the latter is entire 

 or shallowly crenate or lobed, in a younger stage sometimes with 

 a whitish brim. The peripherical portion is frequently more or 

 less indistinctly zonated. The surface otherwise is smooth and 

 most often feebly shining, never with excrescences, but now and 

 then covering up small extraneous objects, and often provided with 

 very small, scaly thickenings not visible to the naked eye, or 

 sometimes getting slightly uneven when emptied conceptacles be- 

 come irregularly effaced by local formations of tissue. Besides,, 

 the surface occasionally here and there is provided with short stria?. 

 I have not seen new crusts formed upon the primary. The colour 

 is much varying, partly nearly according with that of L. polymor- 

 phum partly and most frequently a lighter and more violaceous- 

 purplish shade, or bluish brown, or yellowish brown, American 

 and British specimens frequently being darker than Norwegian and 

 Helgolandian ones. 



The lower, co-axil system of the frond in general is vigorously 

 developed, so far as I have seen occupying about one third of the 

 thickness, partly less partly, however, even more, and its anticlinals 

 partly converge gradually partly more strongly towards the matrix. 

 The cells of the upper thickening layer are on a vertical section 

 arranged in straight rows, and are square or rectangular, including 

 the walls 5—8 (i long and 4—6 p. thick. 



The conceptacles of sporangia are scattered in great numbers 

 over the whole frond, partly nearly conflue.nt i except a rather narrow 



