STKUCTURE OF NACEE. 511 



the well-known Haliotis splendens, — which affords us the oppor- 

 tunity of examining the plaits without any disturbance of their 

 arrangement, and thus presents a clear demonsti'ation 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 resi- 

 duum 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 au undisturbed condition ; 

 and their surfaces then exhibit their iridescent lustre, although all the 

 calcareous matter has been removed from their structure. On look- 

 ing 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 exliibit, 

 are often of the most gorgeous description. If the membrane be 

 extended, however, with a pair of needles, these plaits are un- 

 folded, and it covers a much larger surface than before ; but its 

 iridescence is then completely destroyed. This experiment, 

 then, demonstrates that the peculiar lineation of the surface of 

 nacre (on which its iridescence undoubtedly depends, as originally 

 shown by Sir D. Brewster) is due, not to the outcropping of alter- 

 nate layers of membranous and calcareous matter, but to the dis- 

 position 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 present what may be termed a sub- 

 naereous structure, their polished surfaces being covered with 

 lines indicative of folds in the basement-membrane ; but these 

 folds are destitute of that regularity of aiTangement, which is 

 necessary to produce the iridescent lustre. This is the case, for 

 example, with most of the Pectinidoe (or Scallop tribe), also with 

 some of the Mytilacece (or Mussel tribe), 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 nacre- 

 ous aspect ; and this is the case with the internal layer of by far 

 the greater number of shells, the presence of true nacre being 

 exceptional, save in a small number of families. It is of the 

 inner layer that those rounded concretions are usually formed, 

 which are often found in the interior of shells, and which, when 

 composed of nacreous substance resembling that of the lining of 

 the shell of Avicula, are known as pearls. Such concretions are 

 found in many other shells ; but they are usually less remarkable 

 for their pearly lustre; and when formed at the edge of the 

 valves, they may be partly or even entirely made up of the pris- 

 matic substance of the external layer, and may be consequently 

 altogether destitute of the pearly aspect. The " membranous" 

 shell-substance, some form of which constitutes the internal layer 



