40 THE SHELL. 



and Cypr8ea-(lxi), that the shell only shows externally its last 

 whorl, with, perhaps, a very small portion of its predecessors 

 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 imaginary cone, widest at the base. Besides 

 the almost numberless modifications of form resulting from the 

 degree of obliquity and closeness 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, placing 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 (Physa, Clausilia, etc.). 

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

 dextral will exceptionally produce sinistrally coiled shells, and 

 vice versa^ and this abnormal growth probably is caused by 

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



Whilst the bulk and weight of these shells are composed 

 principally of carbonate of lime, 3^et 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 (conchyolin) 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 removed 

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

 pears and the shell becomes pure carbonate of lime, growing at 

 the same time more and more brittle. 



There are, as before stated, three layers of deposition (of 

 which the middle one is thickest in many cases), each composed of 

 a multitude of plates or prisms, but each differing in the direction 

 of arrangement of these. It will be readily perceived how much 

 this diversity of arrangement adds to the toughness of the shell 

 structure, as no line of fracture can penetrate the entire shell 

 wall, except by the violent breaking across of part of these 

 layers of prisms. 



As to the chemical composition of shells, the conchyolin or 

 organic material is a small, var^ang percentage, carbonate of 

 lime, existing in quantities varying, from Turritella, SS'TO per 

 cent, to Strombus gigas, 99 per cent. There are traces of other 

 constituents, of which carbonate of magnesia is perhaps one of 

 the most important ; it varies from 0*12 per cent, inTelescopium 

 to 0"48 per cent, in Fusus antiquus. Silicic acid has also been 

 detected. C. Schmidt has obtained almost 1 per cent, of phos- 

 phate of lime from the shell of Helix memoralis. The calcareous 

 operculum of Turbo (analyzed by Wicke) contains: Carbonate 

 of lime, 98-*l2 per cent,; organic material, 1-28 per cent. The 



