444 Transformations of Planorbis. [June, 



the last one formed, and the inner, longer layers e' which simply 

 serve to strengthen and support these are laid on by the mantle 

 itself,*/, Fig. 12. 



The mantle border in the gasteropod forms a sort of collar, d+ 

 Figs. 8 and. 12, PI. vn, around the edge of the mantle, through 

 which the creeping disk or foot projects when the animal is ex- 

 panded. The mantle border among the lamellibranchs d+Fig. 18, 

 PI. vn, 1 forms a wider opening, or slit fore and aft, but it serves the 

 same purpose of an aperture for the protrusion of the foot, when 

 this is used as an external organ of motion, Fig. 18, PI. vn. 



Any force which, would confine or interfere with the excreting 

 surfaces of the mantle border, would affect the form of the imbri- 

 cated layers e' which determine the shape of the shell, and thus 

 change or curve the form of the cone. The weight ot the shell 

 itself or gravitation is such a force. If we try to account for the 

 regularity of the spirals, whether bilateral or unsymmetrical, we 

 are struck by the fact of their great regularity of curvature, and 

 that this regularity can only be accounted for as the result of some 

 general and constantly acting physical force, which tends to make 

 the bag-like, straight shell of the young bend first into a bilateral 

 spiral and then into an unsymmetrical spiral. 



The force of gravitation, unless counteracted by great muscular 

 strength, or the equilibrium of a perfectly cone-shaped, erect 

 shell, as in some mollusks, would make the growing shell hang 

 back and weigh upon the hinder portion of the border of the 

 mantle, thus compressing the excreting surface in that quarter 

 and decreasing the breadth of the shell layers built by this part. 



This would make the shell assume the form of a bent cone or 

 the bilateral spiral as in Figs. 24-25, PI. vn. 



The unsymmetrical spiral would be occasioned by any additional 

 inequality in the weight of one side over the other, which could 

 be occasioned by the distribution of the heavier internal organs, 

 particularly of the stomach, ovaries, etc. Any irregularities of 

 weight on one side more than the other would, it is obvious, also 

 compress that side as well as the back part of the mantle border, 

 and tend to narrow the deposits. 



This would occasion a deflection laterally, and we should have 

 what is so commonly the case, a shell bilateral in the young or 

 at the apex becoming by growth unsymmetrical or spiral. 



1 See also Fig. 3, a fresh water clam (Anodonta) thrown widely open. 



