and Observations on Shell and Bone . 
3 21 
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 carbonate of lime ; which, instead of being simply cemented 
by animal gluten, is intermixed 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 ex- 
pect, 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 crustaceous Marine 
Animals. f 
As I was not acquainted with any experiments by which the 
chemical nature of the substance which covers crustaceous ma- 
rine animals had been determined, I was desirous to ascertain 
in what respect it was different from shell, and I began these 
* Some experiments which I have lately made upon 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 composi- 
tion, is exactly similar to shell, and consists of various membranes hardened by car- 
bonate of lime, without the smallest mixture of phosphate. 
f Under this head I have included my experiments upon Echini, Star-fish, Crabs, 
Lobsters, &c. 
