COXSTRUCTIOX OF SHELLS. O 



those of the pith or bark in vegetables. Some pass froin 

 one surface of the layer to another ; others terminate in 

 points midway, marked by delicate transverse striae. A few 

 may be compared to small burnished mirrors, or large 

 nuclear spots. The hinge-tooth of the My a exhibits a very 

 curious variety in large cells occupied by carbonate of lime 

 disposed in a radicated form of crystaUization, resembling 

 that of the mineral called T\'avellite. jN^or less curious than 

 beautiful is the internal layer of different kinds of bivalves, 

 which present a nacreous or iridescent lustre, the result of 

 its surface being varied with a series of grooved lines 

 running nearly parallel to each other. In like manner the 

 well-known Ear- shell, Haliotis sj)lendens, has been ascer- 

 tained to consist of numerous plates resembling tortoise- 

 shell, alternating with thin layers of pearl or nacre, which 

 exhibit, when highly magnified, a series of irregular folds. 

 The iridescent hues are often extremely pleasing ; and if a 

 piece be submitted to the action of diluted acid, till the cal- 

 careous portion of the nacreous layers are dissolved, the 

 plates of animal matter fall apart, each one carrying with it 

 the membraneous residuum of the layer of nacre that belonged 

 to its inner surface. But the nacre and membrane covering 

 some of these horny plates remain undisturbed, and their 



