CURREY, ON FRESH-WATER ALG. 215 
of which fig. 19, by the rupture at the apex, clearly shows 
that the interior is hollow and not solid. A power of fifty 
diameters is quite insufficient to show the constituent cells, but 
their nature is clearly seen in fig. 22, which shows a portion of 
a more advanced frond magnified nearly 700 diameters. It is 
but rarely that specimens such as those in figs. 18 and 19 
are obtainable. The membrane, instead of remaining of a 
globular shape, very soon becomes bent and crumpled into a 
variety of forms, as shown in figs. 20 and 21. The fronds 
are very delicate, and are apt to be put out of shape even 
by the pressure of the thin glass cover, and it is advisable, 
therefore, to examine them in a thin cell, or at all events to 
prevent the direct pressure of the glass cover upon them. 
In process of time the fronds lose altogether their mem- 
branous nature, and when full grown they consist of a gelatin- 
ous mass, with multitudes of small cells imbeddedin it. By 
bringing the margin of a frond into focus, it is seen that the 
gelatinous mass extends considerably beyond the outermost 
cells, and a very slight pressure disperses the jelly and sets 
free the minute imbedded cells. The full-grown fronds are 
usually of irregular shape, but with a length or diameter 
sometimes as great as one fifteenth of an inch or even more. 
It will be seen, therefore, that the Alga just described 
might, at one period, be classed with Monostroma, at a 
later with Palmella. The gelatinous mass doubtless arises 
from the partial dissolution, as well of the walls of the pri- 
mary cells constituting the original membranous fronds, as 
of the walls of the cells produced by the self-division of such 
primary cells. 
6. Clathrocystis @ruginosa, Henfrey.—In figs. 23 and 
24 I have drawn an Alga, which seems to be identical with 
Mr. Henfrey’s Clathrocystis eruginosa, figured im ‘ Trans. 
Mier. Soc.,’ vol. iv, figs. 28—36. It differs somewhat in the 
much greater number of reticulations. In this case, as in 
that of Monostroma roseum, there seems to be a transition 
from hollowness to solidity. In fig. 23 the frond has a re- 
ticulated structure, and having been split open by a clean 
fissure, has the appearance of being hollow in the interior. In 
fig. 24, on the other hand, the green cells seem to be im- 
bedded in solid, soft, and colourless gelatinous matter. Fig. 
25 shows a mass of the component green cells; and fig. 26, 
some of the same cells detached. They vary much in size ; 
the secondary cells in the interior of the larger ones being 
similar to the smaller free cells. ‘The mass of cells was much 
grown over with small colourless Spirilla, as shown in fig. 25; 
this was the case in very many, if not most, of the fronds. 
