XI] OF SEPTA 841 



We do not know how these septa are laid down in an Ammonite, 

 but in the Nautilus the essential facts are clear *. The septum begins 

 as a very thin cuticular membrane (composed of a substance called 

 conchjolin), which is secreted by the skin, or mantle-surface, of the 

 animal ; and upon this membrane nacreous matter is gradually laid 

 down on the mantle-side (that is to say between the animal's' body 

 and the cuticular membrane which has been thrown off from it), 

 so that the membrane remains as a thin pellicle over the hinder 

 surface of the septum, and so that, to begin with, th.e membranous 

 septum is moulded on the flexible and elastic surface of the animal, 

 within which the fluids of the body must exercise a uniform, or 

 nearly uniform pressure. 



Let us think, then, of the septa as they would appear in their 

 uncalcified condition, formed of, or at least superposed upon, an 

 elastic membrane. They must follow the general law, applicable 

 to all elastic membranes under uniform pressure, that the tension 

 varies inversely as the radius of curvature ; and we come back once 

 more to our old equation of Laplace and Plateau, that 



p = r 



^r^v] 



Moreover, since the cavity below the septum is practically closed, 

 and is filled either with air or with water, P will be constant over 

 the whole area of the septum. And further, we must assume, at 

 least to begin with, that the membrane constituting the incipient 

 septum is homogeneous or isotropic. 



Let us take first the case of a straight cone, of circular section, 

 more or less like an Orthoceras ; and let us suppose that the septum 

 is attached to the shell in a plane perpendicular to its axis. The 

 septum itself must then obviously be spherical. Moreover the extent 

 of the spherical surface is constant, and easily determined. For 

 obviously, in Fig. 417, the angle LCL' equals the supplement of 

 the angle (LOU) of the cone; that is to say, the circle of contact 

 subtends an angle at the centre of the spherical surface, which is 

 constant, and which is equal to tt — 2/?. The case is not excluded 

 where, owing to an asymmetry of tensions, the septum meets the 



* See Willey, op. cit., p. 749. Cf. also Bather, Shell-growth in Cephalopoda, 

 Ann. Mag. N.H. (6), i, pp. 298-310, 1888; ibid. pp. 421-427, and other papers by 

 Blake, Riefstahl, etc. quoted therein. 



