﻿BEITISH FOSSIL CEPHALOPODA. 19 



of disturbance are not parallel. The vertical section of the nacreous layer shows, 

 besides the parallel lines of deposit, a number of other lines perpendicular to them, 

 and appearing to divide the layer into a number of narrow prisms (PI. II., fig. 1, A). 

 A horizontal section explains the cause (see PL II., fig. 8). A number of small 

 irregularly shaped dark spots are seen scattered over the surface, which correspond 

 to intervals crossing the laminae, filled with some dark substance. In the outer part 

 of the shell they are very small (fig. 7), but in the inside and on the septa they are 

 better displayed (fig. 8). 



These same two layers, which make up the bulk of the external shell, may 

 be seen also in a modified form in the inner side, where the whorl rests on the out- 

 side of the previous one (PI. II., fig. 1, d, <?). Here the outer layer amalgamates 

 with the black layer supposed to be deposited by the hood (fig. 1, c) : though 

 the characters of the two are perfectly distinct, one being transparent and crystal- 

 line, and the other dark, opaque, and amorphous, there is no proper line of 

 junction, but the outer part of the black layer is perfectly honeycombed by the 

 scattered and connected crystalline particles. These differ from those of the outside 

 of the shell in being closer, and so leaving no dark intervals, except when in the 

 black layer. The interior or nacreous layer differs in having the laminae closer 

 together (fig. 1, e). I have counted 980 of them in the thickness of about one-sixtieth 

 of an inch. At the same time the dark spaces are much larger ; they are seen to be 

 irregular in position in this part, and not so directly transverse ; they send out fine 

 ramifying lines, like those of the lacunae of a bone, only shorter (fig. 8), and their 

 function may be formative, but is not nutritive. Thus the inner and outer layer 

 may be formed by a modification of the same process ; in both secretion takes place 

 at various spots ; in the outer layer the crystalline particles remain distinct, but in 

 the inner layer they amalgamate into laminae, and leave only ramifying lacunae to 

 represent their interspaces. The outlines of the laminae are not absolutely regular, 

 but occasionally aggregate towards a lacuna, or one is lost there. Besides these two 

 layers there is a third, lining the interior of the shell. This is of very small thick- 

 ness, and consists of similar laminae to the nacreous layer (fig. 1,/), and its existence 

 may often therefore be overlooked or doubted, but it may be seen lying between the 

 septum and the inner layer of the shell, with its laminae running in a different 

 direction. They are not, however, so parallel in their outlines, but are thrown into 

 irregular folds by very numerous lines of disturbance. In the acute angles made by 

 the junction of the septa with the circumference of the shell is another deposit 

 (fig. l,g), less transparent than the nacreous layer, but showing very little structure. 

 As this is not moulded on the surface of the mantle, but merely fills the space 

 between contiguous parts, it may be looked on as possibly the result of an excretion 

 which takes up its actual place by " capillary attraction." 



The septa consist of one main layer only, which is formed in exactly the same 



D 2 



