225 
ON THE STRUCTURE OF HILDENBRANDTIA FLUVIA- 
TILIS, Brebiss. 
Bx H. J. Carrer, F.R.S., ETC. 
Communicated by Dr. Gray, F.R.S., of the British Museum. 
* (Puate XX.) 
This Alga was found growing over flints in a little well about three 
.feet in diameter and one foot deep, which encloses the spring or 
source of a freshwater rivulet at a place called “Tidwell,” about a 
mile from Budleigh-Salterton, on the south coast of Devon. It consists 
of an extremely thin thalloid incrustation of a stale-blood or madder- 
brown colour, and of a layer of one, two, or more cells deep, extending 
over areas of two or three inches in diameter, and terminating in an un- 
dulating even edge. The surface-layer presents a tessellated appearance 
of polygonal cells (Fig. 5), arranged sometimes hexagonally, at others 
in lines more or less gyrating from a point, and each cell contains the 
colouring-matter, which gives to the whole expansion the deep-red tint 
above-mentioned. Besides this also, the surface presents a scabrous 
appearance (Fig. 1), produced by the presence of colourless or less 
coloured spots of a circular or eurvilinear form, which remain separate 
or run into each other, having their centres raised, papillary, cracked 
by upward expansion, or excavated, showing, when broken up under 
the microscope, that they are composed of columns of cells, generally 
seven deep, with the uppermost one frequently divided into two or 
four daughter-cells (Fig. 3). Thus separated, too, all the cells of the 
columns are found to be equally colourless, nucleated, and in some in- 
- stances void of contents, i.e. empty. Add to this, that on the surface of 
these light-coloured or papillary projections the cells are seen to be 
somewhat larger than those of the plane or thinner parts of the layer. 
Thinking that the light-coloured circular spots would be found to 
contain elements of fructification like the conceptacles of Hildenbrandtia 
sanguinea, the common marine form, with which H. fluviatilis closely 
agrees in colour when fresh, I examined the scabrous patches or spots 
of the latter under the microscope, both in situ and broken up, but, in- 
stead of the paraphyses and theca common to H. sanguinea, I found 
nothing more than the columns of cells described; thus, H. fluviatilis, 
VOL. 11, [AUGUST 1, 1864.] Q 
