30 MANUAL OF THE MOLLTTSCA. 



give the pearly lustre, wliich appears to depend on minute 

 undulations of the layers, represented in Fig. 23. This lustrQ 

 has been successfully imitated on engraved steel buttons. 

 Nacreous shells, when polished, form "mother of pearl;" 

 when digested in weak acid they leave a membraneous residue 

 which retains the original form of the shell. This is the most 

 easily destructible of shell-textures, and in some geological 

 formations we find only casts of the nacreous shells, whilst 

 those of fibrous texture are completely preserved. 



Pearls are produced by many bivalves, especially by the 

 Oriental pearl-mussel {avicula margaritifera), and one of the 

 British river mussels {unto margaritiferus). They are also found 

 occasionally in the common oyster, in anodonta cygnea, pinna 

 nohilis, mytilus edulis, or common mussel, and in spondylus 

 gcederopus. In these they are generally of a green or rose 

 colour. The pearls found in area noce are violet, and in anomia 

 cepa purple. They are similar in structure to the shell, and, 

 like it, consist of three layers ; but what is the innermost layer 

 in the shell is placed on the outside in the pearl. The iridescence 

 is due to light falling upon the out-cropping edges of partially 

 transparent corrugated plates. The thinner and more trans- 

 parent the plates the more beautiful is the iridescent lustre ; 

 and this is said to be the reason why sea pearls excel those 

 obtained from fresh-water molluscs. Besides the furrows 

 formed by the corrugated surface there are a number of fine 

 dark lines (ttVo i^ich apart), which may add to the lustrous 

 effect. In some pearls these lines run from pole to pole like 

 the longitudes on the globe ; in others they run in various 

 directions ; and in a few the lines on the same pearl have 

 different directions, so that they cross each other. The nucleus 

 frequently consists of a fragment of a brownish-yellow organic 

 substance, which behaves in the same way as epidermis when 

 treated with certain chemical re-agents. Sand is generally said 

 to be the nucleus ; but this is simply a conjecture which has 

 gradually become regarded as a fact ; it is quite the exception 

 for sand to be the nucleus ; as a general rule it is some organic 

 substance. In some districts one kind of nucleus seems to be 

 more common than another ; at least, this is how the different 

 results obtained by observers in different localities may be 

 explained. Filippi [SulV origine delle Perle. Translated in 

 Miiller's Archiv. 1856) found distoma to be the nucleus in many 

 cases ; Kuchenmeister found that the pearls were most abundant 

 in the molluscs living in the still parts of the river Elster, where 

 the water-mites {limnochares anodontce) existed most nume- 



