ORDER PECTINIBRANCHIATA. 67 
that projects inwards, forms, as it were, the commencement of 
a columella, and interposes itself between a fold of the abdo- 
minal sac; the gills are composed of a range of numerous 
filaments, long and slender, like hairs. 
In some of them this lamina adheres to the bottom of the 
cone, being itself bent into a portion of a cone or of a tube, 
and descending vertically. (Patella equestris, L., List. 546. 
38, &c.) 
In others it is almost horizontal, and adheres to the sides of 
the cone, which is marked above by a spiral line, that esta- 
blishes some relation between this shell and that of a trochus. 
(Patella contorta, &c.) 
SIPHONARTA, Sowerb. 
The shell of the siphonariz, which have been recently 
separated from the patellz, at the first glance seems very simi- 
lar to a flattened patella with radiating furrows; but its mar- 
gin projects rather more on the right side, and is excavated 
beneath by a slight furrow, terminating at this projection, to 
which also corresponds a lateral hole in the mantle, for the in- 
troduction of water into the branchial cavity placed on the 
back; at all other points it is completely closed. The respi- 
ratory organ consists of a few small lamelle, arranged in one 
transverse line on the roof of this cavity; the tentacula seem 
to be wanting, the head being merely furnished with a narrow 
veil. (Patella sipho, Sowerb., &c.) 
There are some species in which even this slight appear- 
ance of the canal in the shell is effaced, resembling altogether 
that of a patella, except in its summit, which is behind. 
(Stphonaria tristensis, Sowerb.) : 
SIGARETUS, Adans. 
The shell is flattened, its aperture ample and round, and the 
spire very moderate; its whorls rapidly enlarging, and seen 
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