04 KJELLMAN, THE ALG OF THE ARCTIC SEA. 
Description of the species. External shape. When young, the plant forms an almost 
circular, thin crust on hard objects that lie free on the bottom, especially on shells of 
Balanide, and on muscles and stones. At this stage it resembles a Lithothamnion poly- 
morphum. The crust is slightly rose-coloured, furnished at the edge with feebly marked, 
rounded lobes. At first it is smooth, but it soon gets feeble concentrical ridges especially 
towards the margin. It is closely attached to its substratum. Its surface is enlarged 
by marginal growth and at the same time its thickness increases considerably from the 
centre outwards. If the objects on which the plant has germinated, are of small 
size — to about 10 cm. in diameter — they are completely surrounded by the crust; 
if they are larger, this is the case only partially. As soon as it has attained a 
slight thickness, 2—3 mm., the crust produccs more or less densely crowded, short, 
conical or wart-like, simple protuberances, on which as well as on the basal layer con- 
ceptacles of sporangia are developed in large numbers (pl. 3, fig. 1 and 2). Old spe- 
cimens differ in shape according to the form of the object included. However, they 
approach generally the shape of a sphere or hemisphere, and have a considerable size 
and weight. Such a ball is often 15—20 cm. in diameter. On their lower side, which 
is turned towards the bottom, such individuals are often furnished with a large opening, 
through which the originally included object has fallen out (pl. 2, fig. 2). The thick- 
ness of the crustaceous basal portion varies much in different parts of the same full- 
grown specimen, from one half to one or two cm. (pl. 3, fig. 3). 
The basal crust often puts forth clumsy protuberances or lobes, varying in circum- 
ference, thickness, and height, and bearing, like the rest of the crust, thinly scattered, 
recular, straight, simple, conical, blunt or cylindrically conical processes or branches, 
sometimes very low and wartlike, sometimes higher, 7—8 mm. long, even 5 mm. in 
diameter at the base. Older specimens have a greyish colour with a faint rose-red 
tint. The surface is never smooth, but finely rugged. The crust-like portion of older 
specimens is traversed with numerous, wider or finer passages made by worms, and is 
rich in cavities produced by boring-muscles. 
Structure of the frond. The fracture of the plant is white, sprinkled with small, 
yellowish-brown spots. A section shows these spots to consist of conceptables of spo- 
rangia, that have been grown over. Such are to be found throughout the whole frond, 
in the branches as well as in the basal crust, at certain places very densely crowded. These 
fact point to the thickening meristema of the frond being superficial, overlapping the roofs 
of the conceptables. With regard to structure, this species agrees in the main with 
L. soriferum. However, on a cut parallel with the longitudinal axis of a branch, the 
cup-shaped layers of tissue appear less sharply marked. This results partly from the 
stratification being disturbed by the buried conceptacles, partly from the inner cells of 
each layer being less different in size from the outer ones, than is the case in L. sort- 
ferum (pl. 8, fig. 6, 9). The corners of the cell-rooms are rounded, their walls are thicker 
than in the last-mentioned species. The thickness of the cells in a median section varies 
between 6 and 10 «, their length between 10 and 22 «. In a cross cut of a branch 
the centre is seen to be occupied by a layer of 5—6-angular cells having a thick 
membrane with double contours (pl. 3, fig. 5, 8). This layer passes outwards without 
