SHELL. 



the wider will be the interval between the 

 lines. When the section passes for any dis- 

 tance 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. 

 Herschel* to the surface of a smoothed deal 

 board, in which the woody layers are cut per- 

 pendicularly to their surface in one part, and 

 nearly in their plane in another. Sir D. 

 Brewster appears to suppose -j- that nacre con- 

 sists of a multitude of layers of carbonate 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 animal la- 

 minae, whilst those of the hard calcareous la- 

 minae stand out. If each line upon the na- 

 creous surface, however, indicate a distinct 

 layer of shell-substance, a very thin section of 

 mother-of-pearl ought to contain many thou- 

 sand 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 dilute acid, so 

 as to dissolve its calcareous portion, no such re- 

 petition 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. The mem- 

 brane is usually found to present a more or 

 less folded or plaited arrangement ; but this 

 has generally been obviously disturbed by the 

 disengagement of carbonic acid in the act of 

 decalcification, which tends to unfold the 

 plaits. There is one shell, however, the 

 well-known Haliotis splcndens, which affords 

 us the opportunity of examining the plaits 

 in situ, and thus presents a clear demonstra- 

 tion of the real structure of nacre. This 

 shell is for the most part made up of a series 

 of plates of animal matter, resembling tortoise- 

 shell in its aspect, alternating with thin layers 

 of nacre ; and if a piece of it be submitted to 

 the action of dilute acid, the calcareous portion 

 of the nacreous layers being dissolved away, the 

 plates of animal matter fall apart, each one 

 carrying with it the membranous residuum of 

 the layer of nacre that was applied to its inner 

 surface. It will usually be found that the nacre- 

 membrane covering some of these horny plates 

 will remain in an undisturbed condition ; and 

 their surfaces then exhibit their iridescent lustre, 

 although all the calcareous matter has been re- 

 moved from their structure. On looking at the 

 surface with reflected light under a magnifying 

 power of 75 diameters, it is seen to present a 

 series of folds or plaits more or less regular; 

 and the iridescent hues which these exhibit are 

 often of the most gorgeous description. But 

 if the membrane be extended with a pair 

 of needles, these plaits are unfolded, and it 

 covers a much larger surface than before ; 

 and the iridescence is then completely de- 

 stroyed. This experiment, then, demonstrates 

 that the peculiar lineation of the surface of 

 nacre (on which the iridescence undoubtedly 



* Edinb. Philos. Journ. vol. ii. 

 t Loc. cit. 



VOL. IV. 



depends, as first shown by Sir D. Brewster), 

 is due, not to the outcropping of alternate 

 layers of membranous and calcareous matter, 

 but to the disposition of a single membranous 

 layer in folds or plaits, which lie more or less 

 obliquely to the general surface. 



There are several bivalve shells which pre- 

 sent what may be termed a sub-nacreous struc- 

 ture, their polished surfaces being covered 

 with lines indicative of folds in the basement 

 membrane ; but these folds are destitute of 

 that regularity of arrangement which is neces- 

 sary to produce the iridescent lustre. This is 

 the case, for example, with most of the Pecti- 

 nidce, also with some of the Mytilacece, and 

 with the common Oyster. Where there is no 

 indication of a regular corrugation of the 

 shell-membrane, there is not the least approach 

 to the nacreous aspect ; and this is the case 

 with the internal layer of by far the greater 

 number of shells, the presence of nacre being 

 exceptional, save in a small number of families. 



The membranous shell-substance, some 

 form of which constitutes the internal layer 

 of most bivalve shells, is occasionally traversed 

 by tubes, which seem to commence from the 

 inner surface of the shell, and to pass towards 

 the exterior. These tubes vary in size from 

 about the l-20,000th of an inch, or even less, 

 to about the l-2000th ; but their general 

 diameter, in the shells in which they most 

 abound, is about l-4000th of an inch. The 

 direction and distribution of these tubes are 

 extremely various in different genera. Thus, 

 in Anomia Ephippium they are scantily dis- 

 tributed in the internal nacreous lamina ; 

 but in the yellow outer layer they are very 

 abundant (Jig. 415.), forming an irregular net- 

 work, which spreads out in a plane parallel 

 to the surface. In Cleidothcerus chamoides, on 

 the other hand, the tubes are abundant in the 

 internal layer of the nacreous lining, where 

 they form an intricate but irregular net-work 

 parallel to the internal surface of the shell ; 

 and from this arise a series of straight tubes, 

 which pass nearly at right angles with the 

 surface, at a considerable distance from each 

 other, through the external portion of the 

 nacreous layer, towards the cellular structure 

 which constitutes the exterior of the shell. 

 This, however, they do not penetrate; stopping 

 short as they approach it, just as the tubes of 

 dentine cease at its plane of junction with the 

 enamel. The diameter of the tubes is toler- 

 ably uniform, even when they divaricate ; the 

 trunk not being much larger than either of 

 the branches. In other instances, however, 

 no such net-work is formed, but the tubes 

 run at a distance from each other, traversing 

 the shelly layers obliquely, and are then 

 usually of comparatively large size : this is 

 the case, for example, with some species of 

 Area and Pectuncidus. That these tubes are 

 not mere channels or excavations in the shell- 

 substance, is proved by the fact that they 

 may be frequently seen very distinctly in the 

 decalcified shell-membrane. They often pre- 

 sent, in their beaded aspect, indications of a 

 cellular origin, as if they had been formed 



o o 



