382 Dr. W. B. Carpenter on the Minute Structure of 



manner (fig. 3) . The edge of each fold therefore marks a line ; and 

 the distance of the lines from each other will greatly depend upon 

 whether the cutting-plane coincides with the plane of the folds, 

 or is more or less inclined to them. As the curvatm'e of the layers 

 of shell causes the cutting-plane to traverse them differently in 

 different parts, a single slice of nacre will often show great va- 

 rieties in the disposition of the lines ; just as the same deal board 

 shows the woody layers very differently divided in its different 

 portions. 



The view I have given of the structure of nacre is fully proved, 

 I conceive, by the following fact. It is occasionally possible to 

 obtain a single layer of nacre, the membrane of which, after the 

 complete removal of the calcareous matter by acid, exhibits the 

 true nacreous lustre, so long as the arrangement of its folds re- 

 mains undisturbed ; but as soon as the membrane is extended, 

 so as to obliterate the folds, the nacreous lustre departs. 



I have found approaches to the nacreous structure in many 

 shells, in which the folds are not sufficiently regular for the cha- 

 racteristic lustre to be exhibited. It is easy to understand, there- 

 fore, why there should be a variation in this respect, within the 

 limits of a single genus. Thus in Ostrea there is usually no per- 

 fect nacre, yet there are species which are truly nacreous. On 

 the other hand, in Mytilus there is usually a truly nacreous in- 

 terior, yet there are species in which this is wanting. When so 

 very slight a difference in the arrangement of the folds will pro- 

 duce this variation, it is not surprising that it should occur among 

 the species of the same genus. 



The nacreous and even the subnacreous structure may be easily 

 recognised by the microscope, wherever they occur, both in the 

 recent and fossil state. It sometimes requires a microscope of 

 very good defining power, however, to separate the lines. As it 

 frequently happens that a nacreous structure is thus shown to 

 exist in fossil shells, where, from some peculiarity in the fossiliza- 

 tion, the characteristic lustre does not present itself, I should ima- 

 gine that this microscopic character must frequently be of great 

 importance to the geologist. The absence of any vestige of it 

 was one of the grounds on which I felt justified in determining 

 that the so-called Avicula^ just now mentioned, could not belong 

 to that genus. 



The other peculiar form of plicated membranous structure is 

 that which presents itself in Terebratula and other allied genera 

 of the group of Brachiopoda. It is difficult to give, in words, a 

 description of this peculiarity, but it will be at once understood 

 by reference to figs. 6 and 7, Plate XIV. ; and, when once seen, 

 is always recognisable with facility. The best mode of showing 

 this peculiar structure I have found to be, the detaching very thin 



