Fotms^ Composition, and Structure of Shells, 339 



tlve proportions of these constituent parts. * Mr. Hatchett, 

 to whom we are indebted for all our correct information on 

 this subject, has divided shells into two classes, according to 

 the proportion and state of their animal matter. The first 

 class he names Porcellaneous Shells, since they resemble por- 

 celain, are usually of a compact texture, and have an enamel- 

 led surface, which is often finely variegated. The convolute 

 shells afford good examples of this class. They consist of 

 carbonate of lime, cemented together by so small a portion of 

 albumen, that, when immersed in a dilute acid, the shell is 

 completely dissolved, and not a sensible trace of it left behind. 

 " The shells belonging to the second class are usually 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 fresh-water mus- 

 cles, the oysters, the Hali5tis, and pearly Turbines are 

 examples of this class. " When immersed in acids they effer- 

 vesce, 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 of thin 

 membranous substances, which still retain the form of the 

 shell." These membranes have the properties of coagulated 

 albumen, f 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 (5uccinum undatum), but they are 

 not properly porcellaneous ; while the various land snails 

 leave an insoluble membrane, though they are not perlaceous 

 shells. 



Whence doth the animal borrow these materials, and in 

 what manner doth it apply them to rear up its mansion well 

 fashioned in every part? To the first question, I believe, no 

 satisfactory answer can be given. Some physiologists indeed 

 have analysed the food, and have summed up its constituent 

 item of lime, and there find the earthy material of the shell. 

 But the quantity of lime which the animal takes in this way 

 seems trifling when compared with what is required, and 

 which it actually excretes. 1 know that, in favour of the opi- 

 nion I combat, it has been asserted that the edible snail ( Helix 

 pomatia), which under ordinary circumstances forms a calca- 



* Raspail and Prevost have found that bivalve shells in the first periods 

 of their growth consist wholly, or nearly so, of phosphate of lime, a dis- 

 covery as inexplicable as it was unexpected. 



•j- Thomson's Chemistry, v. p. 554-5. Edinburgh, 1807. 



