SHELLS. 57 



neons shells. To this class belong the various species of valuta, 

 cyprcea, &c. The shells belonging to the second class are usu- 

 ally covered with a strong epidermis, below which lies the shell 

 in layers, and composed entirely of the substance well known by 

 the name of mother-of-pearl* They have been distinguished by 

 the name of mother-of-pearl shells. The shell of the fresh water 

 muscle, the Haliotis iris, the Turbo olearius, are examples of such 

 shells. The shells of the first of these classes contain a very 

 small portion of soft animal matter ; those of the second contain 

 a very large proportion. Hence we see that they are extremely 

 different in their composition. 



1. Porcelaneous shells, when exposed to a red heat, crackle 

 and lose the colour of their enamelled surface. They emit no 

 smoke or smell ; their figure continues unaltered, their colour 

 becomes opaque white, tinged partially with pale-gray. They 

 dissolve when fresh with effervescence in acids, and without leav- 

 ing any residue ; but if they have been burnt, there remains al- 

 ways a little charcoal. The solution is transparent, gives no 

 precipitate with ammonia or acetate of lead ; of course, it con- 

 tains no sensible portion of phosphate or sulphate of lime. Car- 

 bonate of ammonia throws down an abundant precipitate of car- 

 bonate of lime. Porcelaneous shells, then, consist of carbonate 

 of lime cemented together by a small portion of an animal 

 matter, which is soluble in acids, and therefore resembles ge- 

 latin.f 



Patellae from Madeira, examined by Mr Hatchett, were found, 

 like the porcelaneous shells, to consist of carbonate of lime ; but 

 when exposed to a red heat, they emitted a smell like horn ; and 

 when dissolved in acids, a semiliquid gelatinous matter was left 

 behind. They contain, therefore, less carbonate of lime and 

 more gelatin, which is of a more viscid nature than that of por- 

 celaneous shells. 



2. Mother-of-pearl shells, when exposed to a red heat, crackle, 

 blacken, and emit a strong fetid odour. They exfoliate, and be- 

 come partly dark-grey, partly a fine white. When immersed in 

 acids, they effervesce at first strongly ; but gradually more and 

 more feebly, till at last the emission of air-bubbles is scarcely 

 perceptible. The acids take up only lime, and leave a number 



* Herissant, Mem. Par. 1766, p. 22. Hatchett, Phil. Trans. 1799, p. 317. 

 f Hatchett, Phil. Trans. 1799, p. 317. 



R 



