556 PHILOSOPHICAL TRANSACTIONS. [ANNO 1 7Q9» 



and ending with the last formed internal membrane. The animals which inhabit 

 these stratified shells, increase their habitation by the addition of a stratum of car- 

 bonate of lime, secured by a new membrane ; and as every additional stratum ex- 

 ceeds in extent that which was previously formed, the shell becomes stronger n 

 proportion as it is enlarged ; and the growth and age of the animal become denoted 

 by the number of the strata which concur to form the shell. 



Though the haliotis iris and the turbo olearius are composed of the true mother 

 of pearl, I was induced to repeat the foregoing experiments on some detached pieces 

 of mother of pearl, such as are brought from China. These experiments I need 

 not describe, as the results were precisely the same. I must however observe, that 

 the membranaceous or cartilaginous parts of these shells, as well as of the pieces of 

 mother of pearl, retained the exact figure of the shell, or piece, which had been 

 immersed in the acid menstruum ; and these membranaceous parts distinctly ap- 

 peared to be composed of fibres placed in a parallel direction, corresponding to the 

 configuration of the shell. The same experiments were made on pearls ; which 

 proved to be similar in composition to the mother of pearl ; and, so far as their size 

 would enable me to discern, they appeared to be formed by concentric coats of 

 membrane and carbonate of lime ; by this structure they much resemble the glo 

 bular calcareous concretions, found at Carlsbad and other places, called pisolithes. 

 The wavy appearance and iridescency of mother of pearl, and of pearl, are evidently 

 the effect of their lamellated structure and semi-transparency ; in which, in some 

 degree, they are resembled by the lamellated stone called adularia. 



When the experiments on the porcellaneous shells, and on those formed of 

 mother of pearl are compared, it appears that the porcellaneous shells are composed 

 of carbonate of lime, cemented by a very small portion of gluten ; and that mother 

 of pearl and pearl do not differ from these, except by a smaller proportion of car- 

 bonate of lime ; which, instead of being simply cemented by animal gluten, is in- 

 termixed with, and serves to harden, a membranaceous or cartilaginous substance ; 

 and this substance, even when deprived of the carbonate of lime, still retains the 

 figure of the shell. But between these extremes there will probably be found many 

 gradations ; and these we have the greater reason to expect, from the example 

 afforded by the patellae, which have been lately mentioned. Some few experiments 

 were made on certain land shells ; and in the common garden snail I thought that 

 I discovered some traces of phosphate of lime ; but, as I did not find any in the 

 helix nemoralis, it may be doubted whether the presence of phosphate of lime should 

 be considered as a chemical character of land shells.* 



Experiments on the covering substance of crust aceous marine animals. <\ — As I was 



* Some experiments which I have lately made on the cuttle-bone of the shops, have proved, that the 

 term bone is here misapplied, if the presence of phosphate of lime is to be regarded as the characteristic 

 of bone ; for this substance, in composition, is exactly similar to shell, and consists of various mem- 

 branes hardened by carbonate of lime, without the smallest mixture of phosphate. f Under this 



head 1 have included my experiments on echini, star-fish, crabs, lobsters, &c. — Orig. 



