﻿18 BEITISH FOSSIL CEPHALOPODA. 



(fig. 6, a). There are, however, darker bands in the internal substance, due to the 

 greater scarcity of these particles. The external surface of the shell, which has a 

 number of lines of growth parallel to the curvature of the aperture, shows that this 

 layer is gradually deposited by the edge of the mantle. This mantle edge must also 

 be crinkled, for there are an infinitude of spiral lines imbricating in different 

 directions, themselves thrown into crinkles by the lines of growth. On the approach 

 of this layer to the inner one, where the crystalline particles are largest, it is 

 lined on the inner side by a fine network of dark lines, without any regularity, 

 and this is the only separation between the two layers. The inner layer is nacreous 

 to the external view. In a section from the outside to the inside are seen a number 

 of nearly horizontal parallel lines (see PI. II., fig. 1, a), more marked in some places 

 than in others, and irregularly spaced : these are not parallel to the inner or outer 

 surface of the layer, but pass towards the outside of the shell, as we trace them 

 forward, and thus indicate that the nacreous layer has been formed, not at the edge 

 of the shell, but by the surface of the mantle, as a series of deposits lying on each 

 other obliquely to the surface of the shell. This is described by Hyatt as imbricated 

 structure ; it is not only seen on the large scale, where a few of the layers being 

 discoloured render it more conspicuous, but is of the essence of the formation of the 

 layer. For under the microscope each part of the layer is seen to be composed of 

 successive plates from 20 ^ 00 to 30 ^ 00 of an inch in thickness (see PI. II., fig. 7). 

 As these crop out in the interior of the shell, or on a polished surface, they produce 

 a number of parallel lines, which, causing interference of the rays of light falling on 

 them, give the appearance of nacre or mother-of-pearl. This is not well seen in the 

 later part of the shell, because the surface is there covered by a thin lining layer, or 

 by the prolongation of a septum, but the blue and red iridescent colours may be 

 seen in the first two or three chambers, and the ends of the fine plates seen cropping 

 out under a low power (see PI. II., fig. 5). Dr. Carpenter * has expressed the 

 opinion that nacre is due to the folding of a membrane, and not to a succession of 

 deposits. Both, however, in the case of Haliotis splendens to which he refers, and in 

 that of the Nautilus, the nacreous appearance is certainly caused by these simple 

 superposed lamina?, whose whole course may be traced in a vertical section. In a 

 horizontal section they produce a number of slightly curved parallel lines, 2 which 

 are closer or farther apart, according as the section cuts the laminae vertically or 

 obliquely. In places they are thrown out of their direct course (see PI. II., fig. 7, a), 

 to make an acute angle, all the apices of the angles lying on a straight line, just as 

 though the successive laminae had to pass over some narrow obstacle ; these lines 



1 ' The Microscope and its Revelations,' 1862, p. 607. 



2 These curved lines make a thin and perfectly flat section look concave on the side that was so 

 in the shell and convex on the other, just as the undulating lines on a Haliotis makes its section look 

 crinkled. 



