KONGL. SV. VET- AKADEMIENS HANDLINGAR. BAND. 20. N:0 5. 101 



Lithothamnion compactum nob. 



L. fronde Crustacea, initio arete adnata, demum crustis numerosis superimpositis foimatis' usque 2 cm. 

 crassa, e matrice soluta; crusta primaria valida, circa 5 mm. crassa, subnitida, in statu juvenili striis brevioribus, 

 densis, radiatim et concentrice dispositis, nudo oculo inconspicuis, in statu sporangifero foveolis, rainntissimis, 

 creberrimis inaequali, dilute vinoso-purpurea, fiavescente vel albesconte, e cellulis minutis constructa; concepta- 

 culis sporangiferis immersis, demum inuatis, numerosis, circumscriptione globosis vel depresso-globosis; sporangiis? 

 Tab. 6, fig. 8—12. 



Syn. Lithothamnion polymorphum Kjellm. Algenv. Murra Meer. p. 8. 



Description of the species. Habit. If the plant is allowed to develop freely on a 

 plane surface, it forms an almost circular crust, whose extent depends on that of the 

 substratum and whose thickness is comparatively considerable, amounting sometimes 

 to 5 mm. in profile. It increases pretty uniformly in thickness, and thus the periphe- 

 rical portions are not much thinner than the central ones. When young and sterile 

 the crust appears perfectly smooth to the naked eye, as if it were polished. Magni- 

 fying shows, however, the surface to be uneven in consequence of very fine striag 

 partly radiated partly concentric. Older specimens with conceptacles of sporangia have 

 their surface densely furnished with very small, point-like cavities, imperceptible to the 

 naked eye, whose bottoms are formed by the roofs of the conceptacles. These roofs 

 being dissolved in older, dead crusts, the surface is covered with distinct little holes. 

 When young and living, it is feebly wine-coloured, and this is also the colour of the 

 surface in older, living specimens. The fracture of these is however chalky-white with 

 a faint tint of yellow. The young plant adheres closely and firmly to its substratum. 

 Upon the young primary crust new crusts are formed, one upon the other, so closely 

 united to one another, that the limit between them is very difficult to detect on a 

 section. These crust-layers often attain 2 cm. in thickness, and when older detach 

 themselves easily from the objects, other Lithothamnia and stones, over which they 

 have spread (fig. 8). 



Structure of the frond. The lower, co-axil system of the frond is scarcely per- 

 ceptible on a radial section. Like the boundary-layer of the upper system, it has a 

 faint yellowish colour differing from that of the other frond. It consists of rather 

 elongated cells (fig. 10). In the upper thickening-layer of the frond the cells are seen 

 on a radial section to be arranged in straight, well marked rows, square or rectangular, 

 with their greatest extent in the vertical direction of the frond, no more than 10 ^. 

 long and only about 5 f^. thick. The corners of the cell-rooms are scarcely rounded 

 (fig. 10, 11). The surface cells are isodiametrical in a tangential direction with almost 

 circular cell-rooms. Their diameter amounts to 5 fJ-., the thickness of the dissepiments 

 to scarcely 2 ^. (fig. 12). 



Organs of propagation. The conceptacles of the sporangia are always immersed, 

 never rising above the surface. They are to be detected externally only by means of 

 the small cavities that are to be seen above them in the surface of the crust. After- 

 wards they grow down into the frond. They are numerous, rather small, spherical or 

 flattened-spherical. The texture of the roof seems to be easily dissolvable; in dead 

 specimens it is destroyed, in consequence of which the frond is covered with a number 



