60 TRANSACTIONS OF THE WAGNER FREE 



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 wrin- 

 kles, except where the recess of the canal allows them to become more em- 

 phatic, or to a 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 approxi- 

 mate to those of nature. It is difficult, however, to find any artificial tissue 

 which will correspond in elasticity, or capacity for partial self-contraction, to 

 the living tissues concerned in nature. Hence an exact conformity is not to 

 be expected, though the mechanical principles may be reasonably well 

 illustrated. 



The folds or wrinkles of the mantle-edge in the average moUusk will not 

 be visible, as a rule, at the aperture when the animal is living, and the question 

 may be raised as to whether any wrinkling really takes place. To this I may 

 reply that in those forms which have the adductor attached close to the aper- 

 ture, in hardened specimens withdrawn from the shell after immersion in al- 

 cohol, there is generally no wrinkling visible, unless at the anterior and pos- 

 terior commissures ; but in forms with a deep-seated adductor, even the strong 

 contraction caused by alcohol does not eliminate all the wrinkles. It is also 

 noteworthy, in connection with this hypothesis, that in gastropods in which the 

 edge of the shell is free from the edge of the mantle (and ridges due to inser- 

 tion in the tissues, as of Fissurellidea or Chiton, are not to be looked for) there 

 are no species which possess lirs, elongated ridges or plaits, except such as 

 those in which the process here described is practicable. Nodes or " teeth " 

 may be secreted without any " to-and-fro " motion, but there are no case.s 

 where long, internal shelly ridges are produced without the opportunity for 

 such motion. This fact alone seems to me to go far toward a demonstration 

 of the present hypothesis. 



A comparison of specimens will show that the results exhibited agree with 

 marvellous precision with the results called for by the preceding 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. 



All this, I repeat, is purely mechanical and a result of the dynamical con- 

 ditions existing in the shell and related to the mechanical movements of the . 

 animal. With the inception of the plications and lirse natural selection has 

 nothing whatever to do. With their preservation and perpetuation it is 

 legitimately concerned. In what way can it operate ? As to this, without 

 being dogmatic, I may suggest the following explanation: 



The area of attachment of the adductor muscle is small at best, and its 



