16 ENGLAND TO RIO DE JANEIRO CHAP. I 
these stung the person who touched them in the same 
manner as nettles, only much more severely. On the top 
of this bladder was a membrane which he turned either one 
way or the other to receive the wind; this was veined with 
pink, in an uncommonly beautiful manner; in short, the 
whole was one of the most beautiful sights I have seen 
among the mollusca, though many of them are beautiful. 
The floating shells, Helix ianthina’ and violacea, from 
their particularity, also deserve mention. They are to be 
found floating on the top of the water by means of a small 
cluster of bubbles filled with air, composed of a tenacious 
slimy substance, not easily parting with its contents; these 
keep them suspended on the surface of the water, and serve 
as a nidus for their eggs: it is probable that they never go 
down to the bottom, or willingly come near any shore, as 
the shell is of so brittle a construction that few sea-water 
snails are so thin. 
Every shell contains within it about a teaspoonful of 
liquid, which it freely discharges on being touched; this is 
of a most beautiful red purple colour, and easily dyes linen 
clothes ; it may be well worth inquiry whether or not this 
is the purpura” of the ancients, as the shell is certainly found 
in the Mediterranean. We have not yet taken a sufficient 
quantity of the shells to try the experiment, perhaps we 
shall soon. 
Procellaria oceanica differs very little from P. pelagica, 
Linn., but from his place of abode so far south, and some 
small difference in plumage, it is more than likely that he 
is different in species. 
9th. Found two new species of Lepas (vittata and midas) 
on the stern of the ship; they were both sticking to the 
bottom, in company with L. anatifera, of which there was 
great abundance. 
10th. Took plenty of Helix ianthina and some few of 
violacea. Shot the black-toed gull of Pennant; it had not 
} These two species are not Helices, but belong to the genus Janthina. 
? The purple of the ancients has since been proved to have been derived 
from a species of Murex or of Purpura. 
