510 



MOLLUSCOUS ANIMALS GENERALLY. 



FIG. 256. 



scent lustre, which depends (as Sir D. Brewster has shown 1 ) upon 

 the striation of its surface with a series of grooved lines, which 

 usually run nearly parallel to each other (Fig. 256). 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 nacre is carefully 

 examined, it becomes 

 evident that the lines are 

 produced by the croppin 

 out of laminse of shel 

 situated more or less 

 obliquely to the plane of 

 the surface. The greater 



Section of IS acreous lining of Shell of Avicula margantacea ,- -, . /> , -i j 



(Peari oyster). 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 sec- 

 tion 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. HerscheP 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 suppose 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 laminae, whilst those of the hard calcareous laminae stand 

 out. If each line upon the nacreous surface, however, indicates 

 a distinct layer of shell-substance, a very thin section of mother- 

 of-pearl ought to contain many thousand laminse, in accordance 

 with the number of lines upon its surface ; these being frequently 

 no more than l-7500th of an inch apart. But when the nacre is 

 treated with dilute acid, so as to dissolve its calcareous portion, 

 no such repetition of membranous layers is to be found ; on the 

 contrary, if the piece of nacre be the product of one act of shell- 

 formation, there is but a single layer of membrane. The mem- 

 brane is usually found to present a more or less folded or plaited 

 arrangement; but this has generally been obviously disturbed by 

 the disengagement of carbonic acid in the act of decalcification, 

 which tends to unfold the plaits. There is one shell, however, 



1 " Philos. Transact." 1814. 2 " Edinb. Philos. Journal," vol. ii. 



