STRUCTURE OF SHELLS. 431 



The shells of the prismatic crystalline structure exhibit, 

 when broken, a quantity of short fibres perpendicular to the 

 surface : when examined, these are found to be mostly hex- 

 agonal prisms, with a few smaller polyhedral prisms inter- 

 posed. This structure, which has been repeatedly described, 

 is to be observed in the tube of the large Teredo from 

 Sumatra, in the vitreous deposit of Magilus, and in many 

 bivalves, especially those which belong to the families of 

 Mytilidse and Aviculidae, as the Pinna?, Mother-of-pearl 

 shells, &c. ; and it may likewise often be seen in fossil shells, 

 such as the Inoceramus, fragments of which, exhibiting this 

 structure, are frequently found imbedded in flints. 



On the other hand shells of the concretionary structure 

 exhibit, when broken, a nearly uniform texture : they sepa- 

 rate, when heated, into numerous thicker or thinner laminae ; 

 and when digested in weak muriatic acid the lime is dis- 

 solved, leaving a great number of thin plates of animal 

 matter, which retain the original shape of the shell. In 

 general the plates of which these shells are composed are 

 very thin, and closely applied to each other, forming by 

 their union a hard and compact texture. 



The pearly or iridescent lustre appears to be confined to 

 shells of this texture, in which it is very general; a circum- 

 stance which induces me to believe that this lustre depends 

 in a great measure on the thinness and number of the laminse 

 of which the shell is formed.* 



This variety of structure is found to constitute the whole 

 shell of the Anomiae and Placunse ; and to form the inner 

 coat of those shells which have pearly insides, as the Tur- 

 bines, Haliotides, Uniones, &c., as well as the laminar por- 

 tion of the Pinna? and Mother-of-pearl shells. 



When such shells disintegrate, they separate into very 

 numerous thin lamellar scales of a pearly grey colour and 

 silvery lustre. The Chinese are aware of this circumstance, 

 and use the particles of the disintegrated Placuns as silver 

 in their water-colour drawings. I have myself used some 

 of this silvery powder, brought to England by Mr. Reeves, 

 for the same purpose, in colouring the figures of fish with 

 good effect : it is not quite so brilliant as the powdered leaf 

 metal, but it has the advantage of not changing colour by 

 exposure. 



In some shells of this structure the layers are thicker, and 

 the animal matter is deposited in larger quantities, giving 



* The iridescence of the Turbinella prismatica and of Bulimus cactivorus 

 appears to depend upon the texture of tlie periostracum. 



