100. KJELLMAN, THE ALG OF THE ARCTIC SEA. 
the substratum, and does not exhibit any tendency to become circular, as in the pre- 
ceding species? The central parts thicken more rapidly and considerably than the 
peripherical. New crusts may be produced upon others, whereby may be formed crust- 
layers severel mm. thick. The margin of the crust is shallowly crenate with rounded 
lobes. The nature of the surface is determined by that of the substratum; if this is 
smooth, the crust is also smooth and shining when young. Older crusts always become 
uneven and finely rugged on the surface, by growing over and covering up small extra- 
neous objects, and especially on account of the peculiar shape of the conceptacles. Fresh 
fractures of older crusts are of a pure white colour, in younger crusts they are white 
with a faint rose-coloured tinge, at least outwards. The surface is faintly rose-coloured, 
the brim whitish (fig. 11). 
Structure of the frond. A basal, co-axil layer is almost always distinctly and 
vigorously developed, with pretty strongly incurved, anticlinal cell-rows, whose cells 
are about twice as long as thick (fig. 13, 14). The cells of the upper thickening-layer, 
which on a radial section are arranged in distinct rows, that are slightly curved only 
nearest to the median, but otherwise straight, are square or rectangular, with the height 
greatest, 7—9 uw. thick, and even 15 w. long, with thick walls and the corners of the 
cell-rooms rounded (f. 15). The surface cells are angular, 7—10 wv. in diameter, with 
the diaphragms about 3 uw. thick (fig. 16). 
Organs of propagation. The conceptacles of sporangia are very numerous, densely 
crowded both in the internal and external portions of the frond. Hence the name of 
the species is derived. They are not, or scarcely not, raised above the surface of the 
frond, but they are sharply marked on it by their circular or oblong roofs being sur- 
rounded with a strongly prominent, annular border (fig. 12, 17). The roof is almost 
plane or slightly concave, traversed with comparatively few — I have numbered about 
40 — transversely six-sided gelatiniferous canals, whose orifices are surrounded with 
a ring of cells different from the other cortical cells of the roof (fig. 18). 
The sporangia are tetrasporic, slightly club-shaped or cylindrically spindle-shaped, 
from 120 to 185 «., generally about 150 w. long and 45 mw. thick (fig. 19). Sporocarps 
unknown. \ 
Habitat. It occurs on rather open coasts within the sublitoral zone in 5—15 
fathoms water, and seems to grow scattered in small number. Specimens taken at the 
end af August had most of their sporangia void. The development of spores takes 
place probably before that time. 
Geogr. Distr. It belongs to the arctic region of the Polar Sea. Here it has ap- 
parently a pretty wide range. The northernmost place where it is known with cer- 
tainty to grow, is Actinia Bay, about 76° N. Lat. 
Localities: The Kara Sea: local and scarce at several places, as at Uddebay at 
76° 8’ N. Lat., 90°25’ O. Long., Actinia Bay. 
Baffin Bay. A Lithothamnion brought home by Tu. M. Frms from the west coast 
of Greenland seems to me to belong to this species. 
