INSTITUTE OF SCIENCE, PHILADELPHIA. 



59 



the most expanded part, and there will be a tendency to a ridge near the angle 

 of the posterior commissure. Repeated dragging of a shell-secreting surface, 

 thus wrinkled, over a surface fitted to receive such secretion, will result in the 

 elevated shelly ridges which on the pillar we call plications, and on the outer 

 lip lirae, if long, or teeth, if short. The commonly existing subsutural internal 

 ridge on the body of the shell near the posterior commissure will mark the 

 special conditions in that part of the aperture. 



When the secreting surface is thus wrinkled or corrugated longitudinally, 

 the wrinkles and the concave folds between them will be directed in the sense 

 or direction 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 contained. 

 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) tend to maintain the furrows and wrinkles in 

 the same place with relation to the thus-initiated lirse, as these elevated lines 

 are called when on the outer lip ; or plaits, when situated on the pillar. 



The modification referred to generally takes place during resting stages of 

 the animal's growth, since while the animal is rapidly extending its coil the 

 secretions seem to be directed toward the extreme margin, and the general 

 mantle-surface resumes its secretive function (or the latter becomes active) 

 somewhat later, after the formation of a definite varix, or thickened margin ; 

 indicating a resting stage in the animal's career. It is probable also that dur- 

 ing rapid growth there is less compression of the tissues than during the rest- 

 ing stages. The external sculpture and some of the modifications of the 

 aperture are connected with the functions of the extreme edge of the mantle ; 

 those we are at present considering relate more especially to the function of 

 its general surface by which the layer which lines the whorls, the pillar, plaits 

 and lirae are solely secreted and deposited. 



In species with the adductor attached to the pillar near the aperture the 

 wrinkles would be fewer, and their action, if any, confined to the vicinity of 

 the margin of the aperture. The deeper the attachment the greater will be 

 the compression of the secreting surface and the distance over which it is con- 

 stantly dragged back and forth, and the consequent length of the ridges of 

 shelly matter deposited. If the inner or mantle-cone had the whole cavity to 

 itself, it is evident 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 here 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 



