ON THE MICROSCOPIC STRUCTURE OF SHELLS. 11 



all these instances, the decalcification of the shell affords a tolerably con- 

 clusive test of the real nature of the structure ; for the absence of cells in 

 the membranous residuum, coupled with the existence of the corrugations in 

 the membrane itself, clearly indicates its character. 



19. In many other instances the membrane is still more gathered up into 

 plaits or folds, which lie over one another, so that their edges present them- 

 selves as a series of lines, more or less exactly parallel. I shall presently 

 show that the peculiarity of nacreous structure is dependent upon this kind 

 of arrangement ; and that another very remarkable form of it is characteristic 

 of the Terebratulce and their allies. 



20. I am at present inclined to believe that a great part of the appearances, 

 which are attributed by Mr. Gray to the rhomboidal crystallization of the 

 carbonate of lime, are really due to the corrugation or plication of the base- 

 ment-membrane ; for there may be noticed in the disposition of the folds, 

 exactly that variation between the different layers, which Mr. Gray has pointed 

 out as resulting from the different directions of the crystallization. Thus in 

 Cyprcea and its allies, the three layers of shell are easily made to come into 

 view in the same section, and it is then seen that the corrugations of each 

 layer cross those of the adjoining one. A different explanation has been 

 offered however by Mr. Bowerbank ; and until I have examined the subject 

 afresh, I avoid expressing a positive opinion on the subject. 



VI. Nacreous Structure. 



21. The superficial aspect of nacre (or mother-of-pearl), and the optical 

 phaenomena which it presents, have been examined and described by Sir D. 

 Brewster* and Sir John F. W. Herschelf . My inquiries into its structure 

 will enable me, I think, to give a more satisfactory description of its forma- 

 tion than has yet been offered ; and also to explain some of the optical phae- 

 nomena, which have not yet been fully accounted for. 



22. When a thin layer of nacre is submitted to the microscope, its surface 

 is seen to be marked with numerous delicate lines, which traverse it with 

 greater or less regularity : sometimes these lines are almost straight, and 

 run nearly parallel to each other at tolerably regular intervals ; whilst in other 

 parts of the same specimen they are seen to follow a more irregular course, 

 and to diverge widely from each other (fig. 17). Sir J. Herschel has not 

 unaptly compared this appearance to that of 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. These lines are seen on the 

 natural interior surface of the nacre, and no polishing obliterates them. Their 

 distance from each other is extremely variable ; I have seen them only l-7500th 

 of an inch apart ; but they are usually in much less close proximity. 



23. When the nacre-lines are carefully examined, it becomes evident that 

 they 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 obviously be ; whilst the less the angle 

 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. 



24-. As far as I can understand Sir D. Brewster's idea of the structure of 

 nacre, he appears to me to suppose, that it consists of a multitude of layers 

 of carbonate of lime alternating with animal membrane, and that the pre- 



* Philosopliical Transactions, 1814 ; and " Optics " in Lardner's Cabinet Cyclopsedia, 

 pp. 115-120. 

 t Edinburgh Philosophical Journal, vol. ii. 



