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 tete tres large et tres 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 epais, dans 

 lequel la coquille est presque entierement cachee." This is 

 really the case in this species, but by no means to such an 

 extent in N. nitida. 



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 front of the shell ; when at rest it is withdrawn, and forms a 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 



