434 ABSORPTION OF THE SHELL. 



thin and ti'ansparent ; and when these septa are minutely 

 examined, they will be found to consist of only a, single one 

 of the three plates of which all such shells are originally 

 composed, which plate will be seen to agree in structure 

 with the inner one of the three. On tracing these septa 

 to the outer whorl it will be observed that every part of 

 them, during the various stages of growth of the shell, has 

 been once a part of the outer whorl ; and since we know 

 from experience that the outer whorls of the you.ng Olive 

 and young Cone are as thick, in proportion to their size, as 

 those of the adult, there is little reason to doubt that each 

 of these septa was originally formed of three plates, in the 

 same manner as the other parts of the shell. That this was 

 actually the case, and that the part remaining is the con- 

 tinuation of the inner plate, the other two plates having been 

 removed by absorption, may be proved by the fact that the 

 other two layers of the same volutions are distinctly visible 

 on the exposed part of the spire, and on the front part near 

 the pillar, while they are deficient only on the thin part of 

 the septa ; and if the outer half of the penultimate whorl, 

 or rather the half-whorl just within the mouth, be examined, 

 the two outer layers will be found to be there, only in part 

 absorbed, leaving a shelving edge directed towards the cavity 

 of the shell.* 



A still stronger instance of the absorption of the septa 

 may be observed in the shells formed by some of the land 

 Mollusca, as, for instance, the Auriculidas. In the young 

 shells of this family the septa which separate the whorls are 

 incomplete, and twine nearly parallel to each other. As the 

 shells increase in size, the later formed septa become much 

 more oblique and broader, and at length completely separate 

 the cavities of the whorls. When, however, the animals of 

 many of the species, especially those of the Melampi, ap- 

 proach maturity, the whole of the septa except the outer 

 half of the penultimate volution are absorbed, leaving a 

 simple cavity in the hinder part of the shell. On further 

 examining the remains of the septum, it will be found that 



* It is probable that some Bernard crabs have also the faculty of dissolv- 

 ing shell, for it is not unusual to find the long fusiform shells (such as Fusus, 

 Fasciolaria and Tudiinella), which are inhabited by these animals, with the 

 inner lip and great part of the pillar on the inside of the mouth destroyed, 

 so as to render the aperture much larger than usual. I have never seen this 

 erosion except in dead shells which had been inhabited by Hermit crabs ; 

 but it does not occur in all that are so tenanted, for I have also observed 

 these animals occupying the shells of Fusi, &c., in which the lip was in its 

 usual state. 



