KINE TOGENESIS. 



257 



the posterior commissure. Repeated dragging of a 

 shell- secreting surface, thus wrinkled, over a surface 

 fitted to receive such secretion, will result in the ele- 

 vated shelly ridges which on the pillar we call plica- 

 tions ; and on the outer lip lirse, if long, or teeth, if 

 short. The commonly existing subsutural internal 

 ridge on the body of the 

 shell near the posterior com- 

 missure will mark the spe- 

 cial conditions in that part 

 of the aperture. 



''When the secreting sur- 

 face is thus wrinkled or cor- 

 rugated longitudinally, the 

 wrinkles and the concave 

 folds between them will be 

 directed in the sense or di- 

 rection in which the body 

 moves in emerging from or 

 withdrawing to the whorl. 

 The summits of the convex 

 wrinkles will be appressed 

 more or less forcibly against 

 the shell -wall exterior to 

 them in which they are con- 

 fined. The semi-fluid, limy 

 secretion of which the shell- 

 lining is built up, exuding from the whole surface of 

 the mantle, will be rubbed away from the lines of the 

 summits of the wrinkles and tend to accumulate in 

 lines corresponding to the concave furrows between 

 the wrinkles. This secretion hardens rapidly and 

 these lines would become somewhat elevated ridges 

 which would by their presence (when once initiated) 



Fig. 56. Fits us par His Con. the 

 body whorl opened, displaying 

 non-plicatft columella. From Dall. 



