404. Dr. F. Miiller on Balanus armatus. 
Ova.—0°17 millim. long, 0°09 millim. thick. In the larve 
I find nothing remarkable; they are very like those of Tetra- 
clita porosa. 
Affinities —The nearest ally of Balanus armatus is B. tri- 
gonus. Indeed, whether the former might not better be regarded 
as a mere variety, and indicated as B. trigonus, var. armatus, 
can only be decided by comparison with numerous examples 
of B. trigonus from various localities. But it may be cited, in 
favour of its specific title, that B. trigonus has hither been 
found only in the Indian and Pacific Oceans, and not in the 
Atlantic, and only on the shells of mollusca and on wood, but 
not in sponges—that in B. trigonus the shell is usually shal- 
low and ribbed, and the mouth has entire margins and is 
almost equal-sided, whilst in B. armatus the shell is usually 
abruptly conical and smooth, and the mouth always distinctly 
toothed and pentagonal—that the scuta are narrower in B. ar- 
matus—and that the armature of the third pair of feet, which 
was never missed in B. armatus, and iudioetl caught the eye at 
the first glance, is not mentioned by Darwin in B. trigonus, 
any more than the strong tooth on the peduncle of the fifth 
pair, which is always present in B. armatus. 
The shells seated upon Carzjoa, when predominantly deve- 
loped in length, and especially when the base somewhat pro- 
jects, sometimes resemble in general appearance the species 
living on Gorgonie, which, in Darwin’s work, form the sec- 
tion B of the genus Balanus; but this resemblance is merely 
the consequence of the similar mode of adhesion, and scarcely 
the sign of any near affinity. In other respects Darwin’s re- 
marks upon the affinities of B. trigonus apply to our species.’ 
Signification of the armature of the cirri.—A similar arma- 
ture of the cirri with spines and points, although not so strongly 
developed, occurs in other Balanti. In individual examples 
of B. improvisus, var. assimilis, these spines, elsewhere di- 
rected upwards, even occur directed downwards and backwards, 
as in B. armatus, on the outside of the joints of the third and 
fourth pairs of feet. This armature of spines and points occurs 
almost exclusively on the surfaces turned towards the margin 
of the opercular fissure, as on the outer surface of the middle 
pairs and on the dorsal surface of the last pair. In this posi- 
tion they cannot serve for the seizure of any prey, but scarcely — 
for any other purpose than the cleansing of the fissure. In 
fact, in living animals, we see that the cirri of the third and 
fourth pairs, the outer surface of which is particularly richly 
spinous, are those which pass closely along the margin of the 
opercular fissure during their protrusion and retraction. 
Now, that it is exactly in spongicolar species, otherwise by 
