226 SHELL ITS COMPOSITION. 



at last the emission of air-bubbles is scarcely perceptible. 

 The acids take up only lime, and leave a number of thin 

 membranous substances, which still retain the form of the 

 shell." These membranes have the properties of coagulated 

 albumen.* But the distinction between these two classes 

 holds good only in extreme cases ; for there are many shells 

 which are intermediate, and stand on debatable ground. 

 The compact bivalves dissolve in the menstruum entirely, 

 as does also the Common "Whelk (Buccinum undatum), but 

 they are not properly porcellaneous ; while the various land- 

 snails leave an insoluble membrane, though they are not 

 perlaceous shells. 



Mr. Hatchett thought that he discovered some traces of 

 phosphate of lime in the shell of the Common Garden-snail 

 (Helix aspersa) ; but as he did not find any in that of the 

 Helix hortensis, it may be doubted whether the presence 

 of phosphate of lime should be considered as a chemical 

 character of land-shells. f According to Raspail and Pre- 

 vost bivalve shells, in the first periods of their growth, 

 consist wholly, or nearly so, of this latter earth, a dis- 

 covery as inexplicable as it was unexpected. 



It has been made a question from what source the 

 animal obtains the earthy material of its shell. Some, con- 

 sidering the vastness of the quantity required, or rather eli- 

 minated, by the millions of testaceous mollusca now living, 

 and that have lived through all ages, and the enormous 

 extent of the formations that have resulted from the de- 

 position of their remains, have been led to ascribe to the 

 animal a power to produce or generate lime by some in- 

 terior, though yet unknown process, regulated by the all- 

 controlling influence of the living principle. In proof of 

 the existence of such a power, they remind us that the 

 proportion of lime contained in sea-water, or river-water, 

 and in their food, is obviously too small to furnish all that 

 is excreted ; and moreover it has been ascertained that 

 snails confined, so that during a whole summer they have 

 had no access to any preparation of lime, yet did secrete 

 and form very considerable portions of shell. J It is dif- 



* Thomson's Chemistry, v. 554, 555. Edinb. 1807. 



* Phil. Trans, abridg. xviii. 556. 



$ To the instances formerly given, I may add another, furnished by Sir 

 John G. Dalyell. " I have seen snails which were kept months in water 

 only, void very sensible excretions, and increase the size of their shells, 

 though these continued uncommonly transparent, nor could the excretions 

 be the remains of food in the stomach, for the snails had never ate ; they 

 were young, and I had bred them from the egg." SPALLANZANI'S Tracts, 

 trans, i. xliii. 



