1894.] Cause of Folds; Shell of Gasteropoda. 913 
an elastic inner surface, with the result that it cannot recoil 
from the former and that a certain uniformity of size and 
direction is imposed upon the wrinkles, except where the 
recess of the canal allows them to become more emphatic, or 
toa less degree, the posterior angle permits a slight expansion. 
The mechanical principles involved may be readily illustrated 
by the experiment of pulling a handkerchief through the neck 
of a bottle, or funnel, followed by a cork in the center. Of 
course, the more nearly the apparatus conforms to the form 
and twist of a spiral shell the more nearly the results will ap- 
proximate to those of nature. It is difficult, however, to find 
any artificial tissue which will correspond in elasticity, or ca- 
pacity for partial self-contraction, to the living tissues con- 
cerned in nature. Hence an exact conformity is not to be 
expected though the mechanical principles may be reasonably 
well illustrated. 
A comparison of specimens will show that the results exhi- 
bited agree with marvellous precision with the results called 
for by the preceeding hypothesis, based on the dynamical 
status of the bodies concerned, their motions and secretions. 
The agreement is so complete as to amount to a demonstration, 
though in certain cases there may be complications which 
need additional explanation. 
A point which may be noted in regard to the Volutide, to 
which my attention was called by Mr. Pilsbry, is that in this 
group the mantle is greatly extended and there would be more 
of it to be wrinkled than in such forms as Buccinum, etc. It 
may be added that the forms in which we note the beginning 
of plaits for this family, many of them, such as Liopeplum 
and Volutomorpha, had the mantle so extended as to deposit 
a coat of enamel over the whole shell, as in the modern Cyp- 
reea, so that here we have an additional reason why plication 
should be emphasized in this group. 
Of course, as before noted, the mechanical principles are the 
same in any group of gastropods, but among those in which 
the wrinkling is confined to the region of the aperture or those 
shells which are lirate or dentate as opposed to plicate, several 
other principles come into play which may be briefly referred 
