28 Mr, Gray on the Structure of Pearls. [Jan. 



oyster, Avicula Margaritifera* so that they are formed like 

 that shell of transverse laminEe, and they consequently exhibit a 

 plate of lustre on one side which is usually Hat, and are sur- 

 rounded by brilliant concentric zones, which show the places of 

 the other plates, instead of the even beautiful soft lustre of the 

 true pearls. 



Some time ago in examining the shells in the British Museum, 

 I observed a specimen of Barbala pUcala,-'\' with several very 

 fine regular shaped semiorbicular pearls of most beautiful water, 

 and on turning to their superb collection of pearls, I found 

 several fragments of the same shell with similar pearls, and on 

 the attentive examination of one of them, which was cracked 

 across, I observed it to be formed of a thick coat consisting of 

 several concentric plates formed over a piece of mother-of-pearl 

 roughly filed into a plano-convex form, like the top of a mother- 

 of-pearl button. On examining the other pearls they all appeared 

 to be formed on the same plan. In one or two places where the 

 pearl had been destroyed or cut out, there was left in the inside 

 of the shell a circular cavity with a flat base, about the depth, or 

 rather less, than the thickness of the coat that covered the 

 pearls, which distinctly proves that these pieces of mother-of- 

 pearl must have^iieen introduced when the shells were younger 

 and thinner ; and the only manner that they could have been 

 placed in this part of the shell must be by the introduction of 

 theni between the leaf of the mantle and the internal coat of the 

 shell ; for they could not have been put in through a hole 

 in the shell, as there was not the shghtest appearance of any 

 injury near the situation of the pearls on the outer coat. 



Since these observations I have tried the experiment of intro- 

 ducing some similar pieces of mother-of-pearl (which may now 

 be truly so called) into the shell of the Anodonta Cygncus and 

 Unio Pictorum, which I have again returned to their natural 

 habitation ; and I am in hopes that some persons who have more 

 convenience, and are better situated for the purpose, will repeat 

 these experiments, especially with the Unio Margai-itifera. I 

 found the introduction of the basis of the pearl attended with 

 very little difficulty, and I should think very little absolute pain 

 to the animal ; for it is only necessary that the valves of the 

 shell should be forced open to a moderate breadth, and so kept 

 for a few seconds by means of a stop, and that then the basis 

 should be introduced between the mantle and the shell, by 



* I have placed tltis slicll with the Jvicultr. as, wlien young, it has the teeth of that 

 genus; and I have seen an old specimen which would scarcely agree with Lamark's 

 " Carilo cdcntulits." 



■f This shell was described and iigured by Dr. Leach in his Zoological Miscellany 

 under the name of Dipsas plicatiii\ but Dipsas has been used as a genus of Annulosa. 

 I have, therefore, adopted Mr. Humphrey's name ; Dr. Leach had changed it to ^/p- 

 piw: pticntua. — It may be the Mytilus plicaiu.i of Solander's MSS. confounded by Dill- 

 wyn with the Mi;tiliis dubius of Gmelin, but the pearls are certainly not " furnished 

 with stalks," as they are described in the Portland Catalogue, p. 59, to be in that shell. 



