228 
p. 63) that the characters pointed out by FosLie are not sufficient for generic di- 
stinction but only for separation of sections beyond the genus. The roof of the 
sporangial conceptacles is frequently scarcely immersed, and it is often, particularly 
in L. levigatum, convex within a feebly elevated border. 
7. Lithothamnion polymorphum (L.) Aresch. 
J. E. Areschoug in J. Agardh, Spec. II, pars 2, 1852, p. 524 ex parte; Rosanoff, Melobes., p. 99; Strömfelt, 
Algveg. Isl., 1886, p. 19, pl. I, fig. 1—3 (sporangia); Foslie, Norw. Forms, p. 86, pl. 17, fig. 17—23 (f. 
tuberculata, f. valida and f. papillata); Mme P. Lemoine, Structure, 1911, p. 87, pl. V fig. 2. 
Phymatolithon polymorphum (L.) Foslie, Syst. Survey, 1898, p. 4, Remarks, 1906, p. 75. 
Eleutherospora polymorpha (L.) Heydrich, Lith. Helgol., 1900, p. 65, Taf. II, fig. 1—14. 
The species forms more or less irregular crusts extended over larger boulders, 
of a thickness of up to 6 mm. As to the structure reference may be made to the 
papers of FosrLıE (1906) and Mme LeE- 
MOINE. The hypothallium is shown in 
fig. 145 A. The cells of the perithallium 
are somewhat variable in thickness, 4—7y, 
in some specimens proportionally narrow, 
4—5 » (fig. 145 B) Mme LEMOINE men- 
tions as an interesting character that the 
starch grains are single, very small and 
grouped at the ends of the cells. This is, 
however, not always so, for I found the 
starch grains up to 3y in diameter, and 
in some parts, frequently the greater part 
of the crust, all the cells were filled with 
starch grains, while they were totally 
Fig. 185. wanting in others, Particularly abund- 
Showing Ihe hypothaliiam and the lower part of the per Aut Starch grains were found in the tissue 
thallium. B, vertical fllaments of another frond with nar- filling out the emptied conceptacles in 
rower cells. C, cells of perithallium with starch grains. 350:1. 2 
the inner part of the crusts. Cells con- 
taining starch grains at the ends of the cells but not in the middle were indeed 
observed, but only as exceptions. Transversal fusions between the cells of the peri- 
thallium occur here and there. 
The increase in thickness of the crust normally takes place by continued growth 
of the perithallium, which may show several zones limited by horizontal, but some- 
what irregularly running lines. In older crusts a more complex structure may be 
found, the frond being composed of two or more crusts one over the other, each 
with a particular hypothallium. This arises through cessation of growth in thick- 
ness in certain parts of the perithallium, which become overgrown by new crusts 
developing from other parts, of the crust. This structure has been mentioned by 
Mme LEMOINE (Struct. p. 24 and 88, pl. V fig. 2), who appears to consider it as 
