KINE TO GENESIS. 



259 



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 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 re- 

 cess of the canal allows them to 

 become more emphatic, 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 hand- 

 kerchief 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 approximate 

 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 tis- 

 sues concerned 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 exhibited agree with marvellous precision with 

 the results called for by the preceding hypothesis, 

 based on the dynamical status of the bodies concerned, 



Fig. 57. Mitra lineolata 

 Heilprin, the body-whorl 

 opened, showing the pli- 

 cations of the columella. 

 From Dall. 



