ON THE MICROSCOPIC STRUCTURE OF SHELLS. 13 



to the minute ripples which cross the surface of the larger waves. I think 

 that my observations furnish the explanation of these appearances. The 

 lines which mark the edges of the plaits are seldom or never quite even, but 

 are more or less wavy. Of these irregularities, some are caused by the mi- 

 nute scratches or indentations left by the polishing material ; but these may 

 be readily distinguished by the experienced observer ; and there is, besides 

 them, a regular series evidently caused by slight transverse undulations in 

 the plaits themselves, which thus form a secondary series of minute corruga- 

 tions, lying at right angles with the principal plaits. These secondary cor- 

 rugations, however, are seldom deep enough to overlie one another, and 

 hence they exhibit no lined edges. I have been able to detect them very 

 readily in the decalcified nacre-membrane, when it has suffered no exten- 

 sion ; when it has been in the least degree stretched, however, the secondary 

 corrugations are flattened, and the edges of the primary folds become quite 

 straight. The reason why the optical appearances resulting from this arrange- 

 ment cannot (as Sir J. Herschel has remarked) be communicated, like those 

 of the primary series, to surfaces of wax, resin, &c., appears to me to be sim- 

 ply this, that' the folds are not deep enough to overlap each other, and that 

 thus no lined edges are produced ; consequently the corrugations give rise 

 to no inequalities on the polished surface, and cannot communicate any pecu- 

 liar character to substances impressed upon it. 



28. In no nacreous shells that I have examined, have I failed to discover 

 the structure which I have described ; and my examination has comprehended 

 examples, both recent and fossil, from all the tribes in which this chai-acter 

 presents itself. 



29. There are several shells which present what may be termed a sub- 

 nacreous structure, their polished surfaces being covered with lines indicative 

 of folds in the membrane ; but these folds being destitute of that regularity 

 of arrangement, which is necessary to produce the iridescent lustre. This is 

 the case, for example, with most of the Pectinidce, also with some of the 

 MytilacecB, and with the common Oyster. It is easy to understand, therefore, 

 why there should be a variation in this respect within the limits of a single 

 genus. Thus in Ostrea there is usually no perfect nacre, yet thei-e are spe- 

 cies which are truly nacreous. On the other hand, in Mytilus there is usu- 

 ally a truly nacreous interior ; yet there are species in which this is wanting. 

 When so very slight a difference in the arrangement of the folds will produce 

 this variation, it is not surprising that it should occur among the species of 

 the same genus. A want of transparency, also, appears to be one cause of 

 the absence of the iridescent lustre. Thus in a very thin layer of the shell 

 of Ostrea edulis, the nacreous lineation is here and there very characteristi- 

 cally shown ; yet the shell possesses no iridescence, partly in consequence, I 

 am inclined to think, of the presence between its layers of the chalky depo- 

 sits formerly mentioned (§ 1), which can neither transmit nor reflect light. 



VII. Tubular Structure. 



30. All the different forms of membranous shell-structure are occasionally 

 traversed by tubes, which seem to commence from the inner surface of the 

 shell, and to be distributed in its several layers. These tubes vary in size 

 from about the 1 -20,000th to the 1 -2000th of an inch ; but their general dia- 

 meter, in the shells in which they most abound, is about l-^SOOth of an inch. 

 The direction and distribution of these tubes are extremely various in differ- 

 ent shells ; in general, where they exist in considerable numbers, they form 

 a network, which spreads itself out in each layer, nearly parallel to its sur- 

 face ; so that a large p£irt of it comes into focus at the same time, in a section 



