292 



ating bands of crystalline fibres, inclined at different angles in 

 contiguous bands. In the outer layers, the crystalline structure 

 becomes strongly marked with rhorabohedral cleavage. Prob- 

 ably the great hardness of this pearl depends upon the peculiar 

 crystalline arrangement, with a little organic matter binding the 

 whole firmly together. 



Pearls exhibit two principal varieties of microscopic structure. 

 The true or nacreous pearl is formed of concentric lamina? of 

 nacre, and shows a finely furrowed surface, and no radiating lines 

 Avithin. The markings of the nacreous membrane, by which iri- 

 descence is produced, are faintly visible in the sections as very 

 fine undulated and dotted lines. In the second variety of pearl, 

 a prismatic cellular structure occurs. These pearls exhibit well- 

 marked radiating lines, as well as concentric layers. In many 

 specimens of pearl, both varieties of structure are found. The 

 cocoa-nut pearl presents a general resemblance in microscopic 

 characters to the second variety, but differs essentially in the de- 

 tails of structure, as is evident from the sections now exhibited 

 of pearls from pearl oysters and from freshwater clams, — showing 

 the nacreous and prismatic varieties, and combinations of both. 



I cannot find that any species of pearl or other concretion re- 

 sembling this has been described, ]^^or could I learn from our 

 best botanical authorities that any concretion is known to occur 

 in the cocoa-nut. The milk of this nut contains, according to the 

 reported analyses, a little phosphate and malate of lime, but no 

 carbonate ; nor is the carbonate found in any part of the nut. 

 Possibly an analysis of the immature nut might give a different 

 result. The only concretions of vegetable origin which approach 

 this in composition and structure are the cystolithes found in the 

 leaves of Urticace^e, and some other families of plants. These 

 are minute bodies, showing concentric lamination. But they 

 consist of a matrix of successive layers of cellulose, upon which 

 crystalline masses of carbonate of lime are deposited in a kind of 

 efflorescence ; a wholly different mode of formation. 



In the animal kingdom, several kinds of concretions besides 

 pearls bear more or less resemblance to this body in composition 

 and structure ; especially the concretions of carbonate of lime 

 formed in the bladders of herbivorous animals, in which more or 

 less animal matter is always combined with the salt of lime. 



