296 
MR. J. D. MACDONALD ON THE ANATOMY OF MACGILLIVRAYIA, 
found it in other examples, of which those at present under consideration may be a 
variety, if not specifically distinct. 
Before the paper already alluded to was written, although I had examined a con- 
siderable number of species, I never found more than four arms encircling the head, 
but I have since discovered six in a single genus with which I had been long fami- 
liar by external characters (Plate XVI. fig. 18 , representing a member of this genus, 
shows the arrangement of its ciliated arms, &c.). The operculigerous lobe of the 
foot is quite cylindrical and of some length, bearing the peculiar operculum on its 
truncated extremity, the clawed process pointing to the left side. The sucker-disc 
is very small, presenting an anterior and posterior lobe, such as exist in Atlanta, 
in which they differ only in being lateral. 
The two tentacula bear each an ocellus on the outer side near the base, and the 
ciliated arms, in every respect save number, resemble those of Macgillivrayia and its 
congeners. The clawed operculum is developed from a spiral nucleus situate near 
the internal or thickened border ; it seems to be a weapon of defence, which is 
wielded with great dexterity by the little animal, which makes skips and jerks by 
means of its complex foot, after the manner of Nassa or Strornhus. 
It may be well to notice here briefly another interesting member of this diminu- 
tive tribe of Gasteropoda, very commonly met with in the South Pacific, and having 
an almost indefinite range. It resembles a miniature iVafica in many points, including 
both animal and shell. The shell is few-whorled, with small compressed spine and 
ventricose mouth ; the operculum paucispiral and well-marked with the lines of 
growth. The foot of the little creature is not unlike a broad and square-toed shoe, 
receiving or bearing the remainder of the animal and the shell. The shoe-upper, as 
it were, presents two rounded lateral lobes which lie over the anterior part of the 
shell, like the menturn of Natica. 
The little animal creeps on its foot with great rapidity, appearing rather to slide 
along than progress by a vermicular movement, and by spreading out and hollowing 
this organ at the surface of the water, by the same instinct which prompts the fresh- 
water Lyrnneead to form a ready boat of its foot, this shell-protected speck buoys up 
its tiny body, cast abroad, though not lost, in the ocean’s immensity. 
