52 Mr. W. Clark on the lanthmse, Scalarise, 



side by a small white muscle to the base of the groove formed 

 by the neck and skin of the foot, at the centre of which, and 

 under the veil between it and the inside of the front skin of the 

 foot, the yellowish-white proboscis and the end of its inner cy- 

 linder may be seen retracted ; the head veil is shallow, sinuous, 

 or lunated, breaking at the right and left sides, very far apart, 

 into two moderately long, flat, triangular pointed tentacula, red- 

 brown on the upper and outside half, and white on the lower and 

 inside portion, forming at the extreme angles small subcircular 

 auricles. The eyes in this species are so excessively minute as 

 scarcely to be detected ; however, if they really exist, we know 

 their precise position by those of N. nitida, in which they are 

 veiy visible and immersed in the skin at the centre of the ante- 

 rior base of the tentacula, on the white concealed portion, but 

 being always covered they appear to be of little use as organs of 

 vision. The foot is an enormous subcircular disk, much larger 

 in proportion than that of any other pectiuibranchous Gasteropod 

 I am acquainted with : though of one plate, it has the aspect of 

 being formed of four lobes ; the anterior portion, by having its 

 upper skin posteriorly and laterally disunited to form what is 

 called the mentum, appears 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 connexion 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 demi-semicircular yellow 

 corneous paucispiral operculum, which is situated just as far from 

 the posterior extremity as to allow the unoccupied part of that 

 end of the lobe to reflect on and conceal it altogether from view 

 when the foot is deployed ; the operculigerous lobe is then con- 

 tinued on each side the cone of the shell, almost to the mentum 

 or front skin, and forms what M. Deshayes calls " un bourrelet 

 circulaire plus ou moins epais, dans laquelle la coquille est 

 presque entierement cachee." This is really the case in this 

 species, but by no means to such an extent in the N. nitida. 



Authors say that the upper flap of the foot is reflexed on, and 

 keeps the tentacula glued as it were to, the front of the shell ; 

 this is not so; it is never thus reflexed, but often on itself, forming 

 a rounded white rouleau. The fact is, that when the animal is on 

 the march, the upper or coloured skin of the anterior part of the 

 foot spreads itself on the shell, but is not reflexed, supporting 

 the tentacula nearly to their tips, but never otherwise, as when at 



