systematic account of the genus Struvea. 275 
at angles varying, from below upwards, from 90° to about 45°- 
The secondary branches are very regular and so arranged as 
to produce zigzags connecting each pair of primary branches. 
The tertiary branches run parallel to the primary, three or 
four bridging over the V-shaped space between two pinnules, 
and enclosing elongated meshes. 
Quaternary branches are formed here and there. The frond 
of the bleached specimen, which Harvey described, is a good 
deal incrusted with calcareous algae, but this may have taken 
place after the death of the plant. 
In 5 . ramosa the frond has a midrib and pinnae, which are 
rather inconspicuous as their diameters are not much greater 
than that of the pinnules (Figs. 3 a and 3 c). The lowest cell 
of the midrib and the lowest cell of each of the two basal 
branches are elongated in the mature (?) frond. The branching 
in some of the specimens is very regular. Two or four branches 
are given off from the top of each cell of the midrib, and their 
pinnules become attached to one another and to the pinnae 
and midrib, so as to form a reticulum which has usually tri- 
angular meshes (Fig. 3 c). The margin of the frond generally 
has projecting pinnae and pinnules, which probably shows that 
the specimens are not mature. The regularity of the reticulum 
varies very much in the different specimens, and sometimes in 
different parts of the same specimen. Similar irregularity 
is seen in S', delicatula and in S. tenuis, where it is 
sometimes due to injuries, but at other times merely to 
change in the angle of branching or suppression of some 
of the branches, where no injury is apparent. Branching in 
S. ramosa often takes place before the formation of a trans- 
verse wall, as shown in Fig. 3 / , but that septa are ultimately 
formed was proved by examining the ends of filaments which 
had been torn up, as was done in S. plumosa. The tenacula 
are here very much like those of S. plumosa , but, like the 
whole of the frond, they have thinner walls than the latter 
species. The rhizoids are well developed, and very delicate (Fig. 
3 d). By focussing the cell-wall at the apex of the tenaculum, 
three or four pits with granular contents are often seen. They 
