THE SHELL. 19 



Troclms, Stomatia, etc., have both layers, the inner one 

 pearly. 



Nacreous shell substance consists of very thin, superimposed 

 membranes, lying parallel with the surface and impregnated 

 with aragonite. The iridescence of this substance proceeds 

 from the waved or wrinkled close edges of these membranes. 

 The aragonite is of crystalline structure, showing beautiful five or 

 six angled tables. (PI. 2, figs. 8, 9.) 



As porcellanous shell structure is essentially the same in 

 almost all Gasteropods, and the only kind of shell structure in 

 most of them, a few sections of Strombus gigas will give a clear 

 idea of the disposition of the material. (PL 2, figs. 4-6.) 



There are, it will be perceived, three layers of deposition (of 

 which the middle one is thickest in this case), each composed of 

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

 tion 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 wail, 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, varying percentage, carbonate of 

 lime, existing in quantities varying, from Turritella, 88.70 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, in Teles- 

 copiuni to 0.48 per cent, in Fusus antiquus. Silicic acid has 

 also been detected. C. Schmidt has obtained almost 1 per 

 cent, of phosphate of lime from the shell of Helix nemoralis. 

 The calcareous operculum of Turbo (analyzed by Wicke) con- 

 tains : Carbonate of lime, 98.72 per cent. ; organic material 

 1.28 per cent. 



Shells are perceptibly harder than, and will scratch calc-spar. 

 Their specific gravity is somewhat higher than that of Carrara 

 marble, being about 2.15 to 2.85 for the prosobranchiates, and 

 varying but slightly in the pulmonates. 



With reference to the coloring of shells (terrestrial as well as 

 marine species) Fischer points out that as a rule, brilliancy of 

 coloring increases towards the equator. He suggests three 



