338 Mr. Home’s Defcription of 
pinkifh white colour : its outer furface is covered by the brajji- 
ftone in which it is inclofed, and the turnings and windings 
which it makes are very numerous. The end of the (hell, 
which opens externally, riles above the furface of the hone on 
one fide half an inch in height, for about half the circum- 
ference of the aperture, bending a little forwards over it, and 
becoming narrower and narrower as it goes up, terminating at 
lad in a point juft over the center of the opening of the (hell ; 
on the other fide it forms a round margin to the furface of the 
brain-ftone. This part of the (hell is much thicker and 
ftronger than that part which is inclofed in the brain-ftone: its 
outer furface is of a darkifh brown colour ; its inner of a 
pinkifh white. 
The animal, when at reft, is wholly concealed in its (hell ; 
but when it feeks for food, the moveable fhell is pufhed {lowly 
out with the cones and their membranes in a collapfed ftate ; 
and when the whole is expofed, the moveable fhell falls a little 
back, and the membrane round each of the cones is expanded, 
the tentacula at the bafes of the cones having juft room enough 
to move without touching one another. The thin membrane 
which lay between the cones and the inclofng fhell is pro- 
truded in the form of a fold, and lies over the external fhell 
which projeTs from the brain-ftone. 
The membranes have a flow fpiral motion, which continues 
during the whole time of their being expanded ; and the ten- 
tacula upon their edges are in conftant aflion. The motion of 
the membrane of the one cone feems to be a little different 
from that of the other, and they change from the one kind of 
motion to the other alternately, a variation in the colour of 
the membrane at the fame time taking place, either becoming 
a (hade lighter or darker ; and this change in the colour, while 
the 
