598 



STRUCTURE OF SHELLS OF BIVALVES. 



distinct structure when examined in a thin section ; and the resi- 

 duum left after decalcification is usually a structureless ' basement- 

 membrane.' In the MargaritacecB and many other families, how- 

 ever, this internal layer has a Nacreous or Iridescent lustre, which 



depends (as Sir D. Brew- 

 Fig. 309. ster has shown*) upon 



the striation of its sur- 

 face with a series of 

 grooved lines, which 

 usually run nearly 

 parallel to each other 

 (Fig 310.) As these 

 lines are not obliterated 

 by any amount of polish - 

 ing, it is obvious that 

 their presence depends 

 upon something peculiar 

 in the texture of this 

 substance, and not upon 

 any mere superficial ar- 

 rangement. When a 

 piece of the Nacre of 

 the Avicula or Pearl- 

 Mussel (commonly known 

 as ' Mother-of-Pearl') is 

 carefully examined, it 

 becomes evident that the 

 lines are produced by the cropping-out of lamina? 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 ; whilst 

 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, no lines will present 

 themselves on that space. And thus the appearance of a section 

 of Nacre is such as to have been aptly compared by Sir J. Herschel 

 to the surface of a smoothed deal board, in which the woody layers 

 are cut perpendicularly to their surface in one part, and nearly in 

 their plane in another. Sir D. Brewster (loc. cit.) appears to have 

 supposed that Nacre consists of a multitude of layers of Carbonate 

 of Lime alternating with Animal Membrane ; and that the 

 presence of the grooved lines on the most highly-polished surface 

 is due to the wearing-away of the edges of the animal laminse, 

 whilst those of the hard calcareous lamina? stand out. If each 

 line upon the nacreous surface, however, indicates a distinct layer 



* "Philosophical Transactions," 1814.— The late Mr. Barton (of the 

 Mint) succeeded in producing an artificial Iridescence on metallic buttons, 

 by drawing closely-approximated lines with a diamond-point upon the 

 surface of the steel die by which they were struck. 



Section of hinge-tooth of Mya arenaria. 



