204 Conchology. 



when arrived at the last stage of their growth. This period is 

 marked in these shells by a kind of margin of about a line in 

 breadth, which is sometimes turned outwards, although the rest of 

 the shell turns on a regular, spiral line. This reflected margin 

 never appears in land shells, but when they have reached the last 

 period of their growth, and when it is once formed, the animal of 

 some species ceases afterwards to continue the convolutions of its 

 spire. Having now arrived at that period of its growth, when it 

 is fit to perform the act of generation, it protrudes itself more fre- 

 quently from its shell, and each time it returns, a viscid fluid 

 which exudes from its neck, is interrupted and deposited on the 

 external margin of the shell. The bulk which the anterior parts 

 of the body have acquired in consequence of the evolution of the 

 generative organs, which are contained in that part of the body, 

 causes it to press more strongly than formerly on the edges of the 

 opening of the shell, every time it protrudes itself, and gradually 

 forces the particles of testaceous matter which have been recently 

 deposited to the external surface, and in direction quite different 

 from that of the former plane of the spire. A short time is only 

 requisite for the complete formation of this elevation; but after it 

 has been formed, if the animal has the power of continuing the 

 spire on the former plane, the shells which had arrived at a larger size, 

 will exhibit from time to time, if the same process be repeated, 

 longitudinal projecting ribs, convex or bent, exactly similar to the 

 external swelling of the opening of the shell, and analagous to the 

 varices which are seen on some species of marine shells. 



This power of continuing the spire, after the formation of the 

 eminence at the opening, is peculiar to sea shells. No farther in- 

 crease, after it is once formed in land shells takes place. The 

 young of some sea shells, as some species of murex^ also possess 

 this faculty of continuing the growth of shells after the formation of 

 similar elevations, even from the earliest period of their existence, 

 and long before it can be supposed that the organs of generation 

 are evolved. This no doubt depends on some pecuhar structure 

 or organization of the animal; and particularly on those of the an- 

 terior parts of the body. 



Tubercles. Many shells are furnished with tubercles, which 

 are produced by the same organs as the rest of the shell. The 

 fleshy protuberances which are placed on the external surface of 

 the neck of the animals which inhabit them, serve as a mould, 

 and according as there are more or less of these tubercles, while 

 the animal enlarges the turn of the spire, and increases its shell so 

 much, there is the same number of protuberances in the convolu- 

 tion. These protuberances, while they remain on that part of 



