144 CLASS IiJ. GASTEROPODA. ORDER VII. PECTINIBRANCHIATA. 
bubbles, in order that they may sink or swim at pleasure ; we dare not, 
however, venture upon this assertion, as it has not yet been satisfactorily 
ascertained: others have imagined that this vesicular organ is a modi- 
fication of the operculum of mollusks, but we see no trace of analogy 
between two organs which are destined to perform such peculiar and 
distinct offices. 
The Ianthinea, of which only two or three species are as yet known, 
are included in the following genus : 
IANTHINA. 
[ANTHINA, Lamarck. 
Testa violacea, subglobosa, turbiniformis, fragilis ; spira subdepressa, an- 
fractibus aut rotundatis, aut acuto-triangularibus ; apertura integra, 
basi angulata, marginibus disjunctis ; labro tenuissimo, acuto, in me- 
dio, quasi sinu, angulato ; columella elongata, recta, reflexa. 
The peculiarity of structure just described as distinguishing the Ian- 
thine from other mollusks, was first figured by Forskael, a Swedish natu- 
ralist contemporary with Linnzeus ; and it therefore becomes a matter of 
surprise that the shell of Ianthina, which had been known to writers since 
the time of Fabius Columna, should have been arranged by the great 
author of the ‘Systema Nature’ in the same natural division with the 
Helices. "The immense difference in the habits and organization of these 
animals is most striking, and we fear that this is but one of many in- 
stances in which that immortal generalizer might have profited consider- 
ably by the labours of others, in the last edition of his esteemed and com- 
prehensive work. Lamarck soon discovered the necessity of setting the 
Tanthine specially apart by themselves: to him we are indebted for the 
formation of the present genus, and to Cuvier for the demonstration of 
their anatomy. 
In consequence of the abundant violet juices of the Ianthinz, their shell 
