259 



C 



The crust is generally orbicular, scarcely exceeding 2 cm in diameter. In one 

 locality only I found larger crusts expanded over stones; as these crusts also in 

 other respects differred from the others, they will be mentioned separately below. 

 The crusts are 1 to 1,5 mm thick. 



According to F'oslie, the hypothallic layer mainly resembles that of Lithoth. 

 Iceve, being rather feebly developed (Remarks p. 112). I found it, however, always 

 consisting of one layer only, the upright filaments springing out from it in a vertical 

 or nearly vertical direction. The thickness of the perithallic cells varies between 6 and 

 9 /I ; the height 



is generally 

 greater than the 

 breadth, often 

 about double 

 (about 13//), but 

 it may be of 

 the same size or 

 even smaller. 

 FosLiE descri- 

 bes these cells 

 as squarish ; I 

 found them ge- 

 nerally more or 

 less roundish, 

 frequently ap- 

 proaching the 



ellipsoid or 

 globe. They are 

 connected with 

 the cells of the 

 neighbouring fil- 

 aments through transversal pits situated about in the middle of the cells; in trans- 

 verse sections 3 to 5 such pits are seen in each cell (fig. 180 C). The crust is 

 traversed by horizontal limiting lines which are stained intensely blue by hsem- 

 atoxyline; they are often seen crossing the middle of the cells (fig. 180 A). Mme 

 Lemoine did not find such limiting lines in any species of LithophijUum (Struct, 

 p. 28). The cells of the perithallium are frequently filled with starch grains, par- 

 ticularly in the under part of the crust. The surface of the frond is frequently 

 much inclined towards the border, which in vertical section shows a great marginal 

 cell (fig. 180 D). 



The conceptacles of sporangia are completely immersed; they had in the spe- 

 cimens examined a transverse inner diameter of 92 — 116 «; they are generally almost 

 globular in a vertical section. They have a single pore in the middle of the roof 



33* 



Fig. 180. 



Lithophgllum orbiculatum. A, vertical section of crust; B, under part of the same crust. C. 

 horizontal section of crust showing the tranverse pits. D, vertical section of margin of frond. 

 E and F, vertical sections of aberrant specimen (no. .5341), F, showing the hypothallium. 350 : 1. 



