260 THE MICROSCOPE. 



The true pearls are composed entirely of nacre, and are exquisite 

 in their colouring. Their structure is shown highly magnified at No. 2, 

 fig. 113. Some, again, are composed of nacre and prismatic cellular 

 structure ; the centre having the prismatic formation, banded outside 

 by two rings of the true pearly matter — nacre, or " mother-of-pectrl." 



In the shell of th* Terebratvla, or Goach^spring shdl, there are 

 openings surrounded by a series of radiating lines, which at first appear 

 like dark oval spots ; but in a vertical section they will be seen to be 

 perforations or tubes running obliquely from the inner to the outer 

 surface of the shell, and having a series of radiating lines on the edge, 

 as in No. 6, Plate VI. The outer layer has here been removed to show 

 the radiating structure around the perforations. Dr. Carpenter has 

 elaborately .described the Terdiratida in the Philosophical Magazine, 

 1854. 



Not less curious than beautiful is the internal layer of different 

 kinds of bivalves, which present a nacreous or iridescent lustre, the 

 whole of its surface being varied with a series of grooved lines running 

 nearly parallel to each other. One of the most remarkable is the well- 

 known Ear-shell, Haliotus splendens ; this has been ascertained to con- 

 sist of numerous plates, resembling tortoise-shell, forming -a series of 

 hexagonal cells, in the centre of which the stellate pigment is deposited 

 (Plate VI. No, 3), alternating with thin layers of pearl, or nacre ; and 

 this exhibits, when highly magnified, a series of irregular undulating 

 folds, represented in the upper portion of the section. The iridescent 

 lines are often extremely pleasing ; and if a piece be submitted to the 

 action of diluted hydrochloric acid, until the calcareous portion of the 

 nacreous layers be dissolved out, 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 

 folded or plaited surfaces, although divested of calcareous matter, ex- 

 hibit iridescent hues of the most gorgeous description. But if the 

 membrane is spread out with a needle, and the plates unfolded to a 

 considerable extent, the iridescence is no longer seen ; a fact which 

 clearly demonstrates, that the beautiful effects presented by the nacreous 

 portions of shells, commonly called mother-of-pearl, are produced solely 

 by the disposition of single membraneous layers in folds or plaits, lying 

 more or less obliquely to the general surface. 



