'Sl^ Natural Histoyy of MolUiscous Animals : — 



perpendicular fibres themselves are marked with many cross 

 lines, and they are thicliish, and of equal lengths (^), Cuvier 

 says, positively, that they are little hollow columns, placed 

 in a quincunx order {Mem,, vol. i. p. 47.) ; but, according to 

 Dr. Fleming, this is an erroneous account ; and he describes 

 them as " narrow laminae," which, when viewed laterally, ap- 

 pear like fine parallel threads ; but, when examined vertically, 

 are found to be waved, and fold upon themselves. Next the 

 plate they are thin, and not much flattened ; but towards their 

 other extremity they become thicker, striated across, and more 

 folded, with irregular margins,'* (PML qfZool., vol. ii. p. 436.) ; 

 and this description agrees pretty exactly with my own ob- 

 servations. 



2. Pearl is another calcareous secretion of molluscous ani- 

 mals deserving notice. It is secreted only by the fish of 

 bivalves ; and principally by such as inhabit shells of foliated 

 structure, as sea and freshwater muscles, oysters, the Pinnae, 

 &c. A pearl consists of carbonate of lime, in the form of 

 nacre, and animal matter arranged in concentric layers 

 around a nucleus, as it is well shown in our figure {Jig. 106.), 

 2Qg copied from a beautiful engraving ap- 



pended to a paper of Sir E. Home's in 

 the Philosophical Transactions, Each 

 layer is presumed, but I know not on 

 what grounds, to be annual ; so that a 

 pearl must be of slow growth, and those 

 of large size can only be found in full- 

 grown oysters. " It is the nacral lining 

 of the central cell that produces the 

 lustre peculiar to the pearl, which cannot be given to artificial 



* In a previous letter [Letter 5., Vol. III. p. 346.] it is stated that Dr. 

 Brewster had proved the peculiar lustre of the pearl to be the effect of 

 light reflected from the surface ; an opinion first entertained, I believe, 

 by the celebrated Robert Hooke. " Which, by the way, may hint us the 

 reason of that so much admired appearance of those so highly esteemed 

 bodies (pearls), as also of the like in mother-of-pearl shells, and in multi- 

 tudes of other shelly sea-substances ; for they each of them, consisting of 

 an infinite number of very thin shells or laminated orbiculations, cause 

 such multitudes of reflections, that the compositions of them, together with 

 the reflections of others that are so thin as to afford colours (of which I 

 elsewhere give the reason), give a very pleasant reflection of light : and 

 that this is the true cause seems likely, first, because all those so appearing 

 bodies are compounded of multitudes of plated substances ; and, next, that, 

 by ordering any transparent substance after this manner, the like pheno- 

 mena may be produced. This will be made very obvious by the blowing 

 of glass into exceeding thin shells, and then breaking them into scales, 

 which any lamp worker will presently do ; for a good quantity of these 

 scales, laid in a heap together, have much the same resemblance of pearls." 



