XI] TRE SETTA 0¥ NAUTILUS 843 



assumption of elliptical form. In a median section of Nautilus, we 

 see each septum forming a tangent to the inner and to the outer 

 wall, just as it did in a section of the strsiight Orthoceras; but the 



Fig. 418. Section of Xaufilus, shewing the contour of the septa 

 in the median plane. 



curvatures in the neighbourhood of these two points of contact are 

 not identical, for they now vary inversely as the radii, drawn from 

 the pole of the spiral shell. The contour of the septum in this 

 median plane is a spiral curve — the conformal spiral transformation 

 of the spherical septum of the rectihnear Orthoceratite. 



