NATICA. 465 
use as organs of vision. The foot is an enormous subcircular 
disk, much larger in proportion than that of any other pecti- 
nibranchous Gasteropod I am acquainted with: though of one 
plate, it has the aspect of being formed of four lobes ; the ante- 
rior portion, by having its upper skin posteriorly and laterally 
disunited to form what is called the mentum, has the appear- 
ance of a distinct upper and lower lobe; so much so, that the 
upper one has been mistaken for and described as a large, broad 
head, “une téte trés large et trés aplatie;” but its central 
anterior connection with the sole shows that it is an integral 
part thereof. The disk, which is the third portion, is not, 
when the animal is in full action, greatly extended at the 
sides ; it is posteriorly rounded, anteally somewhat constricted 
and more truncate, and at rest forms nearly a flat, oval, sharp- 
edged plateau; on this springs the operculigerous or fourth 
lobe, supporting on the posterior portion a semi-semicircular, 
yellow, corneous, paucispiral operculum, which is situated just 
so far from the posterior extremity as to allow the unoccupied 
part of that end of the lobe to reflect on and conceal it alto- 
gether from view when the foot is deployed ; the operculigerous 
lobe is then continued on each side the cone of the shell, 
almost to the mentum or front skin, and forms what M. Des- 
hayes calls “un bourrelet circulaire plus ou moins épais, dans 
lequel la coquille est presque entierement cachée.” This is 
really the case in this species, but by no means to such an 
extent in N. natida. 
On the march the upper flap of the foot is spread on the 
front of the shell; when at rest it is withdrawn, and forms a 
white rouleau that covers the lower part of the tentacula, and 
obscures the eyes situate at their bases, in those species in 
which they are not obsolete. The upper part of the anterior 
division of the foot is marked with fine dark, longitudinal, 
rather close lines or streaks; behind, it is of a pale brown or 
drab; the reflexed operculigerous lobe is white, and the entire 
disk of the sole pale yellowish-white. The animal has the 
power of stowing entirely out of sight the enormous foot ; 
it does so very deliberately, and closes the aperture effectually 
2H 
