MOLLUSCA: THEIR SHELLS 57 



sions, we will examine with a low magnifying power. 

 Each of its surfaces has a sort of faceted, or honeycombed 

 appearance, and the broken edges, which even to the 

 naked eye appear fibrous, are seen to resemble a number 

 of basaltic columns. "The shell is thus seen to be com- 

 posed of a vast number of prisms, having a tolerably uni- 

 form size, and usually presenting an approach to the hex- 

 agonal shape. These are arranged perpendicularly, or 

 nearly so, to the surface of the lamina of the shell ; so that 

 its thickness is formed by their length, and its two surfaces 

 by their extremities. 7 * 1 



The inner layer of such shells is remarkable for pos- 

 sessing in different degrees the property of reflecting rain- 

 bow-like colors, often with great delicacy and splendor; 

 and this is termed nacre, or familiarly " mother-of-pearl." 

 This iridescent lustre depends, as Sir David Brewster has 

 shown,* upon a multitude of grooves, or fine lines, which 

 run in a very waved pattern, but nearly parallel to each 

 other, across the surface of the nacre. "As these lines are 

 not obliterated by any amount of polishing, it is obvious 

 that their presence depends upon something peculiar in 

 the texture of this substance, and not upon any mere su- 

 perficial arrangement. "When a piece of nacre is carefully 

 examined, it becomes evident that the lines are produced 

 by the cropping out of laminae of shell, situated more or 

 less obliquely to the plane of the surface. The greater 

 the dip of these laminae, the closer will their edges be; 

 while the less the angle which they make with the surface, 

 the wider will be the interval between the lines. When 

 the section passes for any distance in the plane of a lamina, 



1 Carpenter, "The Microscope," p. 590. "Phil. Trans." 1814. 



