210 
not distinctly visible in the dried material; they are therefore in many cases not 
shown in the illustrations, or if so, only in small numbers, though as a matter of 
fact, they are always present, or have at any rate been so. I mention this point, 
as Mme Lemoine states that the cells are in some cases connected by open chan- 
nels, (Struct. p. 35) and that in other instances, neither pits nor channels were found 
(l. e. p. 37). As illustrations of the latter, the writer in question cites Lithothamnion 
leve and L. norvegicum; I can here refer to my figs. 129 and 143, where the pits 
are shown. 
Pits between cells belonging to different filaments are found in the Danish 
species only within the genus Lithophyllum, where the cells in the perithallium form 
transverse layers, in which they lie at equal height, and are then connected by 
pits with all the cells in the same layer, with which they are in contact. This has, 
it is true, been known before, but the importance of the fact as a systematic 
character has not been sufficiently emphasized. The character in question would 
in particular seem to afford an excellent means of distinguishing between the genera 
Lithophyllum and Melobesia, which otherwise closely resemble each other. Unfortu- 
nately, I have not been able to ascertain how these pits arise; they are formed at 
an early stage, and I must presume that they originate in a similar manner to the 
secondary pits in the Rhodomelaceæ etc., though I have not been able to demon- 
strate the co-operation of nuclei in the process, probably owing to insufficient fixing 
and staining of the material. 
In all other genera (where, as we have seen, no such transverse pits are found) 
the cells possess another means of entering into connection with cells in other 
filaments, viz. by forming an open communication between them, the separating 
wall being partially dissolved. These fusions, which were first described by Ro- 
SANOFF, are of common occurrence in the Danish species of the family which do not 
belong to the genus Lithophyllum. 
Where the cells lie densely packed and the walls are thin, the fusions make 
themselves apparent merely by the fact that the longitudinal walls are partially 
dissolved (Lith. Lenormandi fig. 133 D); where the cell walls are thicker, on the 
other hand, a distinct connecting channel of varying length is seen. These appear 
both in the hypothallium and in the perithallium, and may very often take place 
between more than two cells. They are particularly easy to distinguish in the basal 
layer of Melobesia and in the central tissue of the upright, branched Lithothamnia. 
In the latter, they often form characteristically curving partially branched bodies, 
which may embrace almost all the cells in the central tissue (fig. 139). In the peri- 
thallium also, however, of the mentioned Lithothamnia, they may be extraordinarily 
frequent (Lith. calcareum, fig. 144, etc.). Fr. SCHMITZ, who has investigated these fusions 
with regard to the behaviour of the nuclei, found in 1880 (Untersuch. über die 
Zellkerne der Thallophyten. Sitzungsber. der niederrhein. Gesellsch. f. Natur- u. 
7 Of the species mentioned below, they appear to be lacking only in Choreonema Thureti, where 
the vegetation organs are highly reduced (cf. Minder 1. c.) and in M. minutula (fig. 172). 
