﻿20 BRITISH FOSSIL CEPHALOPODA. 



manner as the inner layer of the shell (fig. 1, A). In the closeness of the laminas 

 and in the size of the dark lacunas, it is intermediate between the corresponding 

 layers of the convex and concave part of the shell; but the laminae, instead of 

 being oblique, are parallel to the surface of the septum. Hence the septum is 

 not nacreous in the usual sense, there are no outcropping edges on its surface ; but 

 the opaque lacunas are seen, as it were, floating in a transparent medium when it is 

 viewed as an opaque object. The peculiar lustre of a septal surface is not indeed 

 like that of nacre, but more like that of a true pearl. The actual surface is 

 corrugated by a very minute network of straight lines, running between the ends of 

 the lacunas ; but they are very difficult to see, and the lustre may be due rather to 

 the reflection from the opaque lacunas — especially as it may be very closely imitated 

 by artificial pearls — in which there are also opaque particles floating in a transparent 

 ground-mass. On reaching the siphuncle the laminas curve round into the neck, 

 but only the earliest half of the series reaches the apex. In this neighbourhood the 

 lacunas are more numerous and larger. Each septum is lined on either side by a 

 loose amorphous deposit, of which the one on the convex side is the thicker, and is 

 of sufficient tenacity to have the appearance, when cracked, of a torn membrane ; it 

 passes at the sides of the shell into the deposit lining the concavity of the last 

 preceding septum. 



The essential feature in this structure of the shell, so far as it may be applied to 

 the study of fossil forms, is that the external layer is indivisible, while the internal 

 layer is divisible into an infinity of laminas, which are oblique in the shell but 

 parallel to the surface in the septa, and which show a tendency to split more freely 

 along certain lines ; the inner layer being also distinguished from the outer by the 

 presence of lacunas and the absence of radiate crystallisation. 



That the shells of the Orthocerata and other Nautiloids were in like manner 

 composed of two layers is easy of demonstration in a well-preserved shell — such as 

 Orthoceras annulatum of the Silurian, or 0. attenuatum of the Carboniferous ; but 

 further than this there is but little certainty. The whole of the shells that 

 have passed under my examination have suffered so much mineral crystallisation, 

 that their original structure is nearly if not entirely obliterated. In the outer 

 layer the crystalline parts are clear and better defined, and no subdivisions are 

 seen. In the inner layer, on the contrary, the crystals are all spotted, as if by the 

 remains of such vacuities as the lacunas, and the thickness is subdivided by cracks 

 nearly parallel to the boundary between the two layers, which doubtless corre- 

 spond to the easy lines of division in the Nautilus ; but I have not been 

 able to demonstrate the laminas. The septa have a structure similar to that of 

 the inner layer. 



It is very common to meet with specimens whose surface has been " skinned," 

 that is, the outer layers of the shell have peeled off; sometimes the ornaments of the 



