Dr. F. Miller on Balanus armatus. 401 
- The compartments of the Shell.—The tubes which penetrate 
the walls are tolerably wide; in the uppermost part they are 
completely filled, without transverse septa. The inner surface 
of the walls is ribbed longitudinally, usually throughout, but 
sometimes only below. The radii are externally smooth and 
shining, with a fine striation in two directions, the one parallel 
to the septa, the other to the sutural edge. The former is al- 
ways more distinct; it is not caused by prominence of the 
septa. In the radii of the rostrum and of the lateral pieces 
this striation is nearly perpendicular to the walls of the lateral 
pieces; in the radii of the carino-lateral pieces it is perpendi- 
cular to the wall of these pieces. Within, the radii, where 
they do not lie upon the ale, are often finely ribbed by the 
projecting septa; these ribs are generally very distinct, but 
sometimes scarcely perceptible, and sometimes the radii are 
quite smooth internally. In the septa of the radii, the inter- 
stices of which are closely filled up to the suture, but often 
distinguished by their reddish colour from the white septa, I 
have been unable to detect any distinct denticulation. The 
sutures of the ale are smooth. The sheath has a sharp edge 
ead freely below. 
_ Base.—The base is porous; only in very rare cases it 
projects beyond the lower margin of the shell. Even in the 
animals domiciled in sponges the cement-tubes are well deve- 
“eg whilst in Acasta they were not detected by Darwin. 
After the base has been treated with acids, they appear as 
colourless empty tubes. Their ramifieation differs in different 
animals, but is exactly, the same in.the different older and 
younger tubes of the same animal, so that the branches of 
each younger circle run parallel to those of the older inner 
ones. Not unfrequently cecal diverticula occur. At the 
margin of the base, which is rarely got under the microscope 
in a good state of preservation, I have seen the cement-tubes 
dividing into very fine reticulated branchlets, such as Darwin 
deseribes and figures in Balanus tintinnabulum (Balanide, 
pl. 28. fig. 4a). 
_ Mouth—The upper lip (fig. 10) has three approximated 
teeth on each side of the central notch. The mandibles 
have four distinct teeth; the fifth is sometimes entirely de- 
ficient, but is usually to be distinguished as a small tubercle 
above the lower angle of the mandible. On one occasion I 
found in the same animal the fifth tooth uncommonly dis- 
tinctly developed on one side, whilst on the other it was en- 
inity, as though a nerve entered the cord; between the bristles and the 
chitinous cords there seems to be a sort of articulation. 
