ON MOLLUSCA. 157 
bouring genera, which constitute the family of the siphono- 
branches; and even in those of the pulmobranch family, where 
it really merits the name of collar, it is very long and very 
narrow, in the cones, the olives, the porcelaines, where it is 
constituted of two lobes, more or less unequal, and which may 
sometimes greatly pass the aperture of the shell, and be curved 
over it, so as to envelop it totally. Finally, the aperture of the 
mantle may be oval or circular, as in the symmetrical or un- 
symmetrical cervico-branches. In the naked or almost naked 
cephalous mollusca, the mantle being very thick in its whole 
extent, or a very little more so on its edges, or otherwise 
covered with tubercles, as in doris, peronia, tritonia, and even 
in limax, the projecting edges nevertheless pass the foot, so as 
to resemble a species of large buckler. 
In the lamellibranch acephalous mollusca, whose body is 
usually very much compressed, the mantle constantly very 
slender, if we except towards the edges, is divided into two 
great lateral lobes, equal, or nearly so, which fall back on each 
side of the body, compress it between them, and often very 
much exceed it. ‘This is an arrangement pretty much analo- 
gous to that of the porcelaines; and it is here that this part of 
the envelope really deserves the name of mantle. Always 
united in a greater or less extent along the dorsal line, the 
lobes of the mantle of the lamellibranches may be separated in 
all the rest of their extent, as in the oysters ; half separated, 
as in unio, cardium, venus; are well joined, so as to consti- 
tute a sheath, opening only in front and behind, as in solens 
and many other genera; or finally, form a sac, pierced only 
with two posterior apertures, approximated as in the ascidie, 
or more or less distant, as in the biphores, in which the mantle, 
in its external stratum, becomes almost cartilaginous. 
The edges of the aperture of the mantle in the cephalous 
mollusca are often simple ; that is to say, without elongations, 
