size, plentifully borne along the upper or inner edge of the ramuli, a tetra- 
spore usually springing from every articulation. Swdstance in the stem and 
branches cartilaginous, flaccid in the ramuli. Colour a dark vinous red, 
inclining to purple, or sometimes to brown; staining fresh water carmine. 
Under the microscope the colour of the ramuli is a clear crimson lake. 
The most robust and bushy of all the British Cal/ithamnia, and 
therefore well named Arbuscu/a. The main stem is often upwards 
of a line in diameter, and divided into several stout branches, 
densely clothed with finely divided ramuli. The colour is always 
very dark, varying from brownish to a more or less vivid vinous 
purple. No species can well be confounded with the present, 
except, perhaps, very luxuriant specimens of C. spongiosum, but 
the microscopic characters of that species are extremely different. 
Formerly Cal. arbuscula was confounded with Dasya arbuscula, 
a mistake which could only arise from a very hasty exami- 
nation of very imperfect specimens, for independently of generic 
character the plants are very different. In the Dasya the ramuli 
are dichotomous, and here they are pinnated, and far more 
densely crowded. | 
C. arbuscula is extremely abundant on the western coasts of 
Ireland and Scotland, and has been found on several parts of 
the east of Scotland; but is unknown in the east and south of 
Ireland, and has not, that I am aware of, been found in England. 
It delights on the most exposed rocks and the roughest water, 
and very commonly grows on the shells of Mytilus rugosus, mn 
places where it is left dry for some hours each tide. 
Fig. 1. CALLITHAMNION ARBUSCULA, growing on Mytilus rugosus :—the natural 
size. 2. Lesser branch clothed with plumules. 3. Segment of a small 
branch. 4. Plumule with tetraspores. 5. A ramulus and tetraspores 
from the same. 6. Plumule with favelle. 7. Favelle from the same :— 
all magnified. 
