﻿26 BEITISH FOSSIL CEPHALOPODA. 



at this part contains both layers, which are perfectly continuous, and show no signs 

 of the cicatrix having ever been perforated. The outer of the two layers becomes 

 very thick when traced in the section into the concavity of the umbilicus (fig. 4, b), 

 and its junction with the nacreous layer becomes very opaque, so as to look black by 

 transmitted light (as represented by Hyatt), but it is not really so. This thickened 

 part has so similar a structure to that of the exterior of a " cuttle bone "as to suggest 

 that it is the rudimentary representative of that part, and also of the Belemnitic 

 " guard." It has almost, if not altogether, died away by the time that the shell has 

 curved round into contact with the convex side. The inner layer is also thicker at 

 the same spot as the outer, but thins out to the cicatrix, the inner layer of which 

 juts, like a subsequently formed septum, upon its inner side (fig. 4, c), proving that 

 this inner layer was formed subsequently, and is in point of fact the first septum. 

 The most external layer of the cicatrix and of the thickened part is an extremely 

 thin black deposit, which is afterwards developed only on the inner side, where it 

 forms the dark layer seen on the earlier whorls (fig. 4, d). 



In extinct Nautiloids the cicatrix is sometimes absent, sometimes it is round, but 

 more often, especially in coiled shells, it is elongated in the plane of symmetry as in 

 the recent form. Not many of the Silurian species show their initial cap, but there 

 are some which have the cicatrix, as will be noticed, in their place. "When there is 

 no cicatrix, the last semblance of a reason for imagining an earlier nucleus is taken 

 away. The edges of the cicatrix are often swollen, and the area is of comparatively 

 very small size : Barrande supposes it may have been for the passage of some 

 ligament, or the junction of some floating apparatus. Its closure, however, by an 

 undisturbed band of double-layered shell, the outer one continuous with that of the 

 rest of the shell, seems to exclude the idea of its having ever been open. He admits 

 that a study of the embryology of other molluscs might suggest a more probable 

 explanation ; and when we remember that the shells of Gastropods x commence in 

 what is known as the ; ' shell-groove," the step is not a long one to conclude that 

 the cicatrix marks the shell-groove of the Nautilus. I anticipate that when an 

 embryo Nautilus shall be discovered, its first indication of shell will occupy the area 

 now marked out by the cicatrix. 



The surface of the nucleus beyond the cicatrix is sometimes smooth, but more 

 often it is marked with longitudinal lines, as it is in the recent Nautilus (fig. 5, b). 

 These pass often beyond the region of the nucleus for some distance, and affect the 

 whole thickness of the shell ; they are therefore doubtless due to a folding of the 

 mantle. To these longitudinal lines are often superadded concentric ones, of 

 rather irregular character, which produce in this way a network : these may be 

 considered as lines of growth. The characters of the nucleus are independent of 



1 See Kay Lankester, " Development of Pond Snail," Quart. Journ. Micr. Soc. 1874 ; and " Develop- 

 mental History of Mollusca," Phil. Trans. 1875. 



