258 PRIMARY FACTORS OF ORGANIC EVOLUTION. 



tend to maintain the furrows and wrinkles in the same 

 place with relation to the thus-initiated lirae, 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 se- 

 cretions seem to be directed toward the extreme mar- 

 gin, and the general mantle-surface resumes its secre- 

 tive function (or the latter becomes active) somewhat 

 later, after the formation of a definite varix, or thick- 

 ened margin ; indicating a resting stage in the animal's 

 career. It is probable also that during rapid growth 

 there is less compression of the tissues than during the 

 resting-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 abductor attached to the pillar 

 near the aperture, the wrinkles would be fewer, and 

 their action, if any, confined to the vicinity of the mar- 

 gin of the aperture. The deeper the attachment, the 

 greater will be the compression of the secreting sur- 

 face and the distance over which it is constantly 

 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 there the mass 

 of the solid and elastic foot and external body comes 



