NACREOUS SHELL-SUBSTANCE. 



599 



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 frequently no more than 

 1 -7500th of an inch apart. But when the Nacre is treated with 



Fig. 310. 



Section of Nacreous lining of Shell of Avicula margaritacea (Pearl-oyster). 



dilute acid so as to dissolve its calcareous portion, no such repeti- 

 tion 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 arrangement is found in Pearls ; which are 

 rounded concretions projecting from the inner surface of the 

 A vicula, 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 en- 

 tirely made-up of the Prismatic substance of the external layer, and 

 may be consequently altogether destitute of the pearly character. 



463. There are several Bivalve Shells which present what may 

 be termed a Sub-Nacreous structure, their polished surfaces being 

 marked by lines, but these lines being destitute of that regu- 

 larity of arrangement which is necessary to produce the Iridescent 



