18 THE SHELL. 



very small portion of its predecessor visible on the spire. On 

 the other hand there are genera in which the whorls are not at 

 all in contact, and where the axis becomes itself an imaginative 

 cone, widest at the base. Besides the almost numberless modifi- 

 cations of form resulting from the degree of obliquity and close- 

 ness of the spiral, the direction of the latter may be mentioned 

 as another factor in producing modification. In most spiral 

 shells the spire normally curves to the right, that is to say, plac- 

 ing the shell with its apex turned from the observer and its 

 aperture in view, the latter will be found on the right hand. In 

 others the volutions proceed in the opposite direction with such 

 regularity as to be eminently characteristic of some species and 

 genera. However, in certain genera, it is found that species 

 normally dextral will exceptionally produce sinistrally coiled 

 shells, and this abnormal growth probably is caused by disturb- 

 ance of the relations of the embryo with its initial shell. 



Whilst the bulk and weight of shells are composed principally 

 of carbonate of lime, yet they have always an organic basis, 

 which is first developed, and then gradually impregnated with 

 the lime. If the latter be removed by the use of acids the 

 organic residuum (conchy olin) still retains the shape of the shell, 

 forming a sort of membranous framework. It is this organic 

 basis which maintains the life of the shell, for, the animal re- 

 moved, as in beach-worn or fossil species, the conchyolin soon 

 disappears and the shell becomes pure carbonate of lime, growing 

 at the same time more and more brittle. Many of the long spiral 

 shells, such as Bulimus decollatus, some Pupas, Truncatella, 

 Melania, etc., withdraw the body from the earlier whorls in the 

 course of growth, and partition off the unoccupied space with a 

 shelly plate : in such cases the unoccupied whorls become brit- 

 tle and are soon broken off. We must, therefore, believe that 

 the shell is vitalized, or rather, that its vitality is maintained by 

 simple contact of its organic basis with the living animal. 



There are two very distinct types of shell structure, the cel- 

 lular or porcellanous and the membranous or nacreous. In 

 bivalve mollusks the former is the outer and the latter the inner 

 layer, in most cases, but in univalves the shell is usually porcel- 

 lanous only ; although a few of the holostomata, such as Turbo, 



