910 The American Naturalist. : [November, 
Now let us consider the dynamics of the case. We have, 
reduced to its ultimate terms, a twisted shelly, hollow cone, 
sub-angulate or even channelled at two extremes correspond- 
ing to the canal and the posterior commissure of the body and 
outer lip. Inside of this we have a thin, loose epithelial cone, 
the mantle, of which the external surface especially toward the 
margin, is shell-secreting ; lastly, inside of the mantle-cone we 
have a more or less solid third cone, consisting of the foot and 
other external parts of the body of the animal, which can be 
extended beyond the mantle-cone outwardly, as the mantle- 
cone can be beyond the shell-cone. The body-cone and the 
mantle-cone are attached at one of the angles of the shell-cone 
some distance within the opening of the spiral of the latter. 
The two outer cones constitute a loose, flexible funnel within 
a rigid, inflexible funnel, while the body-cone forms a solid, 
elastic stopper inside of all. 
What will happen according to mechanical principles 
(which can be tested by any body with the simplest apparatus) 
when the mantle-cone is withdrawn into a part of the shell- 
cone too small for the natural diameter of the contracted 
mantle-cone? It must wrinkle longitudinally. Where will 
the wrinkles come? They will come at the angles of the shell- 
cone first; they will be most numerous toward the aperture, 
since toward the aperture the mantle-cone enlarges dispropor- 
tionately to the caliber of the shell, owing to its processes, the 
natural fold of the canal, etc., etc.; the deepest and strongest 
wrinkles will be over the pillar, owing to the fact that the 
attachment of the adductor prevents perfect freedom in 
wrinkling, and the groove of the canal will mechanically in- 
duce the first fold in that vicinity. The most numerous small 
wrinkles will be near the aperture opposite the pillar, because 
of the mantle-edge this is the most expanded part, and there 
will be a tendency to a ridge near the angle of the posterior 
commissure. Repeated dragging of a shell-secreting surface, 
thus wrinkled, over a surface fitted to receive such secretion, 
will result in the elevated shelly ridges which on the pillar we 
call plications, and on the outer lip lire, if long, or teeth if 
short. The commonly existing subsutural internal ridge on 
