174 



THE MICROSCOPE AND ITS REVELATIONS. 



Fig. 389. 



series of grooved lines, which usually run nearly parallel to each other 

 (Fig. 389). 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 arrange- 

 ment. When a piece of the nacre (commonly known as ' mother-of- 

 pearl') of the Avicula or ' pearl-oyster ' 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; 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. Her- 

 schel to the surface of a 

 smoothed dealboard, in which 

 the woody layers are cut per- 

 pendicularly to their surface 

 in one part, and nearly in their 

 plane in another. Sir D. Brew- 

 ster (loc. cit. ) appears to have 

 supposed that nacre consists of 

 a multitude of layers of carbon- 

 ate 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 ani- 

 mal 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 hundred laminae, in 

 accordance with the number of lines upon its surface; these being fre- 

 quently 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. This layer, however, is found to present a 

 more or less folded or plaited arrangement; and the lineation of the 

 nacreous surface may perhaps be thus accounted for. A similar arrange- 

 ment is found in pearls; which are rounded concretions projecting from 

 the inner surface of the shell of Avicula, and possessing a nacreous 

 structure corresponding to that of ' mother-of-pearl.' Such concretions 

 are found in many other shells, especially the fresh-water mussels, Unio 

 and Anodon; but these are usually less remarkable for their pearly lustre; 

 and, when formed at the edge of the valves, they may be partly or even 

 entirely made-up of the prismatic substance of the external layer, and 

 may be consequently altogether destitute of the pearly character. 



drawing closely-approximating lines with a diamond-point upon the surface of 

 the steel die by which they were struck. 



Section of nacreous lining of Shell of Avicula marga- 

 ritacea (Pearl-oyster). 



