912 The American Naturalist. [November, 
compression of the secreting surface and the distance over 
which it is constantly dragged back and forth, and the conse- 
quent length of the ridges of shelly matter deposited. If the 
inner or mantle-cone had.the whole cavity to itself, it is evi- 
dent that it could and would infold itself in a manner which 
might not appress its folds against the inner surface of the 
rigid outer or shell-cone. But there the mass of the solid and 
elastic foot and external body comes into play, and by its 
withdrawal inward forces the wrinkled mantle-cone against the 
shell. The mantle is thus confined between a rigid outer and 
Fig. 1. 
Fig. 1. Fusus parilis Conr. a gastropod in which the caliber of the spire 
contracts uniformly within the x ida but which, having a short retractor 
muscle develops no plications on the 
Fig. Mitra lineolata Heilprin, a Qu ot ise similar, , but in which 
the retractor is long and deep seated and in which the axis becomes plicated. 
. 3. Siphocyprxa problematica Heilprin, a gastropod in which the aper- 
ture is contracted and the cavity within ample so that plications are developed on 
the sides of the aperture but not on the axis within 
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